# Any simple checksum methods for CC receipts with tip? **URL:** \ **Category:** Factual Questions\ **Created:** [September 2, 2010, 8:49pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427 "2010-09-02T20:49:08Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![filmore](https://avatars.discourse-cdn.com/v4/letter/f/7993a0/32.png) [@filmore](https://boards.straightdope.com/u/filmore)\ **Post date:** [September 3, 2010, 2:28pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/21 "2010-09-03T14:28:19Z")
> [@Enderw24](#): > > I’m not sure why we need an elaborate system to prove whether something is happening when there’s a simple solution already built in to accomplish what you wish to do. Why do people post stuff like this? I’m not forcing you to come up with a solution yourself. I’m not forcing you to use this solution. If you don’t feel like contributing there are plenty other threads out there. I do use paper receipts today. It would be simpler if I could somehow glance down my CC statement and easily notice a modified entry. I’m searching for that system that I view as simpler. You may not view it as simpler and that’s fine, but please keep that opinion to yourself. ---
**Author:** ![Enderw24](https://avatars.discourse-cdn.com/v4/letter/e/ba9def/32.png) [@Enderw24](https://boards.straightdope.com/u/Enderw24)\ **Post date:** [September 3, 2010, 2:33pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/22 "2010-09-03T14:33:43Z")
> [@filmore](#): > > > [@Enderw24](#): > > > > I’m not sure why we need an elaborate system to prove whether something is happening when there’s a simple solution already built in to accomplish what you wish to do. > > Why do people post stuff like this? I’m not forcing you to come up with a solution yourself. I’m not forcing you to use this solution. If you don’t feel like contributing there are plenty other threads out there. > > I do use paper receipts today. It would be simpler if I could somehow glance down my CC statement and easily notice a modified entry. I’m searching for that system that I view as simpler. You may not view it as simpler and that’s fine, but please keep that opinion to yourself. Thank you for deliberately cutting out the portion of my response that actually answers the question you asked and then claiming that I’m not contributing to your discussion. The point, that I and many others have attempted to tell you, is that no system you devise will work as simply or as effectively as keeping your receipts and comparing it to your statements. And I’d think that a person looking to rationally solve a problem would be interested in the most rational solution there is. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [September 3, 2010, 2:56pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/23 "2010-09-03T14:56:46Z")
How about something that doesn’t require any calculation on the verification end, like taking the digit in the one’s place then the cents are the next two sequential digits? Then back-calculate the tip. When you get your statement you don’t have to calculate a checksum. ```auto Food 32.57 Tip 6.00 Total 38.57 Desired Total 38.90 Revised tip 6.33 ``` FWIW I have used my credit card in restaurants for over 30 years. I enter receipts in Quicken and reconcile my statements (now it’s all electronic and largely automated). Only twice has a credit card amount not matched. Once it was my mistake; I made an arithmetic error in the restaurant when adding up the tip, and the restaurant used the tip amount I had written and corrected the total. The other time the total didn’t match the receipt. The bank just credited me the amount because it was so small, and did not pursue it as fraud. ---
**Author:** ![RaftPeople](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@RaftPeople](https://boards.straightdope.com/u/RaftPeople)\ **Post date:** [September 3, 2010, 2:59pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/24 "2010-09-03T14:59:24Z")
> [@Enderw24](#): > > The point, that I and many others have attempted to tell you, is that no system you devise will work as simply or as effectively as keeping your receipts and comparing it to your statements. And I’d think that a person looking to rationally solve a problem would be interested in the most rational solution there is. But it’s not a valid point. Your point assumes that you can measure, for a different person, which system is simpler, easier and more effective. But you can’t perform that measurement for a different person through a few posts on a message boards. There are variables involved based on personality, lifestyle, volume of transactions, etc. You are ignoring all of this in your calculation. Even on the surface, comparing a bunch of credit card transactions on a statement to a bunch of paper transactions is slower than scanning for certain number patterns. Obviously the trade-off is a little more work up front, but for me personally, the up front work would be easier than matching trans to receipts later as long as the up front method was simple (which it can be). ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [September 3, 2010, 3:15pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/25 "2010-09-03T15:15:45Z")
> [@Bad\_Astronaut](#): > > Plus even if you have your copy of the receipt, the tip portion would just be written in pen, by you. How could you prove that you didn’t just write a different tip on that receipt? Merchants are required to keep their copies of the receipts in case they are audited by the credit card company or if they need to dispute a chargeback. In the case I referred to above, the restaurant provided a copy of the original receipt that I signed and the bank sent it to me and it was clear I had made an error. ---
**Author:** ![Baracus](https://avatars.discourse-cdn.com/v4/letter/b/2bfe46/32.png) [@Baracus](https://boards.straightdope.com/u/Baracus)\ **Post date:** [September 3, 2010, 3:28pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/26 "2010-09-03T15:28:05Z")
> [@Markxxx](#): > > Pay the bill by credit card and leave the tip as cash. > > I was a hotel auditor for years and we always double checked the math on resturant charges for tips. If the server was going to pull anything, they wouldn’t get away with it more than once or twice, unless you had a auditor and accounting department in on it as well. Seeing as the server could still key themselves in a tip, I don’t see how this lessens the need to double check. ---
**Author:** ![mhendo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mhendo/32/3159_2.png) [@mhendo](https://boards.straightdope.com/u/mhendo)\ **Post date:** [September 3, 2010, 3:38pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/27 "2010-09-03T15:38:30Z")
> [@Baracus](#): > > Seeing as the server could still key themselves in a tip, I don’t see how this lessens the need to double check. The difference is that, if you just put a thick line in the Tip section, it’s harder to mess with the credit card slip, and easier to prove fraud. Changing a 3 to an 8, or a 5 to a 6, is relatively easy. But if there’s nothing but a big line there, then writing numbers over it will be much more obvious. ---
**Author:** ![Bad\_Astronaut](https://avatars.discourse-cdn.com/v4/letter/b/e19adc/32.png) [@Bad\_Astronaut](https://boards.straightdope.com/u/Bad_Astronaut)\ **Post date:** [September 3, 2010, 3:44pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/28 "2010-09-03T15:44:53Z")
> [@filmore](#): > > A second part of my question is how to come up with a good checksum. The mistakes I’ve found in the past have been where two numbers are switched. So 23.15 gets entered as 32.15. My simple x+y+z checksum isn’t so good for that type of error since the checksum is the same regardless of the order of the numbers. My checksum for 123 is the same as 321. Is there a simple checksum I can use that will be different if numbers get transposed? By simple I mean an easy math transformation I can do in my head. I believe two checksum digits would be required to accomplish this. For instance, you could repeat the ‘ones’ digit in the ‘tenths’ position, and then use the normal checksum of all digits in the ‘hundredths’ position. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [September 3, 2010, 4:03pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/29 "2010-09-03T16:03:21Z")
> [@filmore](#): > > A second part of my question is how to come up with a good checksum. The mistakes I’ve found in the past have been where two numbers are switched. So 23.15 gets entered as 32.15. My simple x+y+z checksum isn’t so good for that type of error since the checksum is the same regardless of the order of the numbers. My checksum for 123 is the same as 321. Is there a simple checksum I can use that will be different if numbers get transposed? By simple I mean an easy math transformation I can do in my head. Rather than add the numbers directly, add them after multiplying each by a place-dependent mask. If you only want to catch adjacent transpositions, a simple mask will work, such as multiplying alternating digits by 2 before adding them. Example: Value: dc.ba Mask: 12.10 The “0” in the mask just shows that the last digit isn’t involved in the checksum. If you have a $100+ bill, the mask becomes 212.10. $1000+ would use 1212.10 (etc.) So, the sum for a bill becomes: b+2c+d ( +2e + f +2g + …). A bill: 12.43 + 2.46 = 14.89. We want the last digit to be 1 + 4_2 + 8 = 7, so we must remove two cents from the tip. This leaves the total at 14.87. A transposition of, say, the 1 and 4 would imply a checksum digit of 4 + 1_2 + 8 = 4, which would indicate the error, as 4!=7. ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [September 3, 2010, 4:18pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/30 "2010-09-03T16:18:16Z")
> [@kayaker](#): > > > [@Markxxx](#): > > > > Pay the bill by credit card and leave the tip as cash. > > That’s how I’ve always done it. Cash is definitely the best solution. If the whole bill is paid in cash, there’s absolutely no opportunity for restaurant employees to key in anything. Cash also requires the least of the OP’s time, and it saves the restaurant 2% or so in processing fees. ---
**Author:** ![qpw3141](https://avatars.discourse-cdn.com/v4/letter/q/6bbea6/32.png) [@qpw3141](https://boards.straightdope.com/u/qpw3141)\ **Post date:** [September 3, 2010, 4:28pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/31 "2010-09-03T16:28:17Z")
How many restaurants still use this ancient method with CC’s? 😕 Whenever I use a CC in a restaurant the waiter brings the CC machine to the table and I key in the tip. It then prints out the receipt showing the bill, tip and total. So the restaurant never gets hold of my card and I can see that everything’s correct. ---
**Author:** ![Baracus](https://avatars.discourse-cdn.com/v4/letter/b/2bfe46/32.png) [@Baracus](https://boards.straightdope.com/u/Baracus)\ **Post date:** [September 3, 2010, 4:42pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/32 "2010-09-03T16:42:17Z")
> [@qpw3141](#): > > How many restaurants still use this ancient method with CC’s? 😕 > > Whenever I use a CC in a restaurant the waiter brings the CC machine to the table and I key in the tip. It then prints out the receipt showing the bill, tip and total. > > So the restaurant never gets hold of my card and I can see that everything’s correct. Granted that I don’t go out all that often, but I have never once been to a place that did this. ---
**Author:** ![pulykamell](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pulykamell/32/3166_2.png) [@pulykamell](https://boards.straightdope.com/u/pulykamell)\ **Post date:** [September 3, 2010, 4:59pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/33 "2010-09-03T16:59:49Z")
> [@qpw3141](#): > > How many restaurants still use this ancient method with CC’s? 😕 > > Whenever I use a CC in a restaurant the waiter brings the CC machine to the table and I key in the tip. It then prints out the receipt showing the bill, tip and total. I live in Chicago and go out to eat regularly enough (at least once a week). I have never seen this, to the best of my knowledge. ---
**Author:** ![filmore](https://avatars.discourse-cdn.com/v4/letter/f/7993a0/32.png) [@filmore](https://boards.straightdope.com/u/filmore)\ **Post date:** [September 3, 2010, 5:19pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/34 "2010-09-03T17:19:42Z")
> [@Enderw24](#): > > Thank you for deliberately cutting out the portion of my response that actually answers the question you asked and then claiming that I’m not contributing to your discussion. > > The point, that I and many others have attempted to tell you, is that no system you devise will work as simply or as effectively as keeping your receipts and comparing it to your statements. And I’d think that a person looking to rationally solve a problem would be interested in the most rational solution there is. I didn’t mean to call you out specifically. Thank you for the contribution part of your post. You were just one of many posters in this thread who felt the need to post something to the effect of “What you want to do is stupid. Don’t do it at all or do it this other way.” I just don’t understand the point of posting something disparaging like that. ---
**Author:** ![qpw3141](https://avatars.discourse-cdn.com/v4/letter/q/6bbea6/32.png) [@qpw3141](https://boards.straightdope.com/u/qpw3141)\ **Post date:** [September 3, 2010, 5:19pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/35 "2010-09-03T17:19:56Z")
> [@Baracus](#): > > > [@qpw3141](#): > > > > How many restaurants still use this ancient method with CC’s? 😕 > > > > Whenever I use a CC in a restaurant the waiter brings the CC machine to the table and I key in the tip. It then prints out the receipt showing the bill, tip and total. > > > > So the restaurant never gets hold of my card and I can see that everything’s correct. > > Granted that I don’t go out all that often, but I have never once been to a place that did this. > [@pulykamell](#): > > > [@qpw3141](#): > > > > How many restaurants still use this ancient method with CC’s? 😕 > > > > Whenever I use a CC in a restaurant the waiter brings the CC machine to the table and I key in the tip. It then prints out the receipt showing the bill, tip and total. > > I live in Chicago and go out to eat regularly enough (at least once a week). I have never seen this, to the best of my knowledge. Really. I haven’t seen one of those slide action machines anywhere for what seems like years. CC transactions are so much faster and more efficient now. I’m very surprised the on line machines haven’t made their way into restaurants there. ---
**Author:** ![filmore](https://avatars.discourse-cdn.com/v4/letter/f/7993a0/32.png) [@filmore](https://boards.straightdope.com/u/filmore)\ **Post date:** [September 3, 2010, 5:21pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/36 "2010-09-03T17:21:45Z")
> [@Markxxx](#): > > Pay the bill by credit card and leave the tip as cash. I don’t believe this will eliminate the problem. I’m pretty sure the server still enters the total by hand. So whether the bill is this: Food: 23.00 Tip: 4.00 Total: 27.00 or this: Food: 23.00 Tip: ----- Total: 23.00 It’s still up to the server to enter 27 or 23 into some computer. If someone with eyeballs actually checks receipts against totals entered they will catch mistakes, but typically it seems businesses just spot check or not check at all. ---
**Author:** ![filmore](https://avatars.discourse-cdn.com/v4/letter/f/7993a0/32.png) [@filmore](https://boards.straightdope.com/u/filmore)\ **Post date:** [September 3, 2010, 5:23pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/37 "2010-09-03T17:23:37Z")
> [@Pasta](#): > > Rather than add the numbers directly, add them after multiplying each by a place-dependent mask. If you only want to catch adjacent transpositions, a simple mask will work, such as multiplying alternating digits by 2 before adding them. I like this system. Simple enough to do in my head and will catch modification and transposition errors. ---
**Author:** ![filmore](https://avatars.discourse-cdn.com/v4/letter/f/7993a0/32.png) [@filmore](https://boards.straightdope.com/u/filmore)\ **Post date:** [September 3, 2010, 5:39pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/38 "2010-09-03T17:39:21Z")
And I’ll clarify as to why, for me, a checksum is simpler than checking receipts. I’ve been checking receipts for a couple of years and the only mistakes I’ve found have been from the editable receipts (tip). Normal stores which don’t allow modifications to the total have never presented a problem. So checking just the modified receipts will catch most (or all) of the errors. I usually have 20-30 food receipts at the end of the month. It’s a hassle to try to match up each receipt with each entry on the statement. Even if I keep my receipts in order, the transactions on the statement aren’t necessarily in that same order. I often go to the same sets of restaurants. This means have to figure out which of the 5 or so “Moe’s Cafe” receipts goes with which record in the statement. My credit card company lets me download the statement. I can write a program to auto compute the checksums of the entries and kick out the ones which don’t match. By having a checksum, I can run my program and automatically know if any records look wonky. Then I just look for that single receipt to see if there’s a real problem. ---
**Author:** ![pulykamell](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pulykamell/32/3166_2.png) [@pulykamell](https://boards.straightdope.com/u/pulykamell)\ **Post date:** [September 3, 2010, 5:45pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/39 "2010-09-03T17:45:33Z")
> [@qpw3141](#): > > I haven’t seen one of those slide action machines anywhere for what seems like years. What do you mean by “slide action machines”? Do you mean the ones that are heavy and mechanical and carbon copy? Those I (almost) never see. The standard here is you give them the credit card, they go back and swipe it through their credit card machine (the online kind with the magnetic strip reader), and they come back to you with a receipt that says Subtotal, Tip, and Total. You fill in the tip and total lines, sign the receipt, and leave it at the table when you leave. Oh, and take your credit card back, of course. edit: So where do you live? I’ve traveled around a good bit, and I can’t recall coming across any restaurants where they brought the card reader to your table. It’s completely possible I’m forgetting, but the last few trips I’ve been to Wisconsin, Indiana, California, New York, Arizona, etc., I cannot remember any place using this method. Is this something more popular outside the US? ---
**Author:** ![qpw3141](https://avatars.discourse-cdn.com/v4/letter/q/6bbea6/32.png) [@qpw3141](https://boards.straightdope.com/u/qpw3141)\ **Post date:** [September 3, 2010, 6:08pm UTC](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427/40 "2010-09-03T18:08:06Z")
> [@pulykamell](#): > > edit: So where do you live? I’ve traveled around a good bit, and I can’t recall coming across any restaurants where they brought the card reader to your table. It’s completely possible I’m forgetting, but the last few trips I’ve been to Wisconsin, Indiana, California, New York, Arizona, etc., I cannot remember any place using this method. Is this something more popular outside the US? UK (although I think this applies throughout Europe). I didn’t realise that chip and pin had not reached (the whole of) the US. The system here is that they don’t ever get to take your card. They bring a machine to the table and you insert the card and enter your pin. Then you remove the card and they print the receipt. It’s much safer than letting people wander off with your card. The system is also a lot more streamlined and faster to use in use in other retail establishments as there is no signing/checking of signatures. [Previous page](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427.md?page=1) [Next page](https://boards.straightdope.com/t/any-simple-checksum-methods-for-cc-receipts-with-tip/552427.md?page=3)