The 8 acre property I own on the Big Island averages about 200 inches of rain a year.
About how many gallons of water fall on the property each year?
43,446,922.4 gallons. About.
FYI, Google handles calculations with units. You can just type “8 acre * 200 inch in gallons” and get your answer, which is 43.4 million gallons. Wolfram Alpha does this too.
43,445,771 US gallons dammit ninjad because I converted to acre feet by hand.
200 inches is 16.7 feet, so that’s (8 acres) x (16.7 feet) = 133.3 acre-feet, which is a commonly used unit of volume in this context.
1 acre-foot is, according to Wikipedia, approximately 325,853.383688 US gallons. (Yes, that’s absurd precision for an approximate answer, but that’s what they say. 
)
So, about 43.3 million gallons. Like they said.
If you find yourself doing calcs like this frequently, you can do approximations without having to remember too many facts.
First, convert to acre-feet. You have 8*200=1600 acre-inches. Divide by 12 and you have 133 acre-feet.
Second, you remember the fact that a acre-foot is about a third of a million gallons. 133 divided by 3 is… well 1/3 of 100 is 33, and 1/3 of 33 is 11, so that’s about 44. So 44 million gallons.
<2% error for a mental calc isn’t too bad…
To put that in more practical terms in case you’re looking to harvest the rain to put it to use on your property, 200 in/yr will net you about 125 gallons/sq ft.
Mind you, at 200 inches of rain per year, you might be more interested in drainage than capture…
Yes, my catchment tank only holds 10,000 gallons, no wonder its always full.
An acre is 43,560 square feet or 6,272,640 square inches. 8 acres is 50,181,120 square inches and 200 inches of rainfall gives you 10,036,224,000 cubic inches of water. A gallon is 231 cubic inches so you have 43,446.85714 gallons.
At which point I explain to my class the futility of quoting to ten significant figures while being out by a factor of 1000.
1 inch of rain on one acre gets you 27000 gallons so its gonna be around 41800000 gallons.
NOTE: that info was stolen from granpas rain gauge chart from the late 1800’s he was a peanut farmer.
Here’s where the metric system is far easier for mental maths.
If all quantities are expressed in metric units to start with you get
3.237 hectares (or 32370 square metres) by 5 metres.
So simply multiply one by t’other in your head to get cubic metres - 161,850. you can use that as it is to express metric tons of water or multiply by 1000 to get litres. 161,850,000 (or about 43 million of your strange US gallons)
In Europe they tend to express rainfall in “litres per square metre”. This is exactly the same as saying “millimetres”, it just sounds odd to British ears. (If you spread a litre of water evenly over one square metre, it will be 1mm deep.)
In this country which officially went metric in the 1970s, we still get rainfall quoted in inches on radio and TV, although they have stopped giving the Fahrenheit temperatures as well as Celsius. Schools have been exclusively metric for 40 years but many, if not most people still think in imperial.
I suppose that most of us use whatever is more convenient.
golf clap
Canada exclusively speaks metric in the media, so I found I have to do a mental adjustment over the weather when I visit the USA - even though I grew up with imperial.
I used to work at a place where the feed to the plant was measured by a weightometer on a conveyor belt. Every time the staple joint went over the weightometer, the actual reading jumped, so the controller box averaged out the total. The engineer in charge tested it a few times, by the simple expedient of stopping the belt, scraping off a measured amount, and calculating the actual. They had a spreadsheet program (The Good Old Days) that tallied in and out and told them how much was in process and stockpile to approximately a tenth of a pound in millions of pounds. The engineer mentioned that number was particularly meaningful when the input measurement, the conveyor, had an accuracy of +/-5%. Fortunately, the error was all over the map rather than biased in one direction.
Yep, your numbers are only as meaningful as your weakest link.
But everyone is assuming your property is flat. You need to map out the area on a topo map (or equivalent) and figure out the actual “rain cross section” in order to accurate.
Also, please send any excess water you have to CA. 
why would that affect the amount of rain falling or the measurement taken at a rain gauge?
Because the rain might not be falling straight down. In the extreme case, suppose that your property, on a two-dimensional map, encompasses only a narrow strip… but that that narrow strip is right along a sheer cliff face. If the rain is falling into that cliff face, you could get a lot more than your acreage times the rainfall depth.