# What's this unit of measurement: mg^-1 day^-1

**URL:** <https://boards.straightdope.com/t/whats-this-unit-of-measurement-mg-1-day-1/217403>\
**Category:** Factual Questions\
**Created:** [December 6, 2003, 5:50pm UTC](https://boards.straightdope.com/t/whats-this-unit-of-measurement-mg-1-day-1/217403 "2003-12-06T17:50:01Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![TWDuke](https://avatars.discourse-cdn.com/v4/letter/t/e495f1/32.png) [@TWDuke](https://boards.straightdope.com/u/TWDuke)\
**Post date:** [December 6, 2003, 5:50pm UTC](https://boards.straightdope.com/t/whats-this-unit-of-measurement-mg-1-day-1/217403/1 "2003-12-06T17:50:01Z")

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E.g., 14 mg[sup]-1[/sup] day[sup]-1[/sup].

How do you pronounce it? What does it mean? Is it simply milligrams per day? Why the “-1”? Thank you.

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**Author:** ![DarrenS](https://avatars.discourse-cdn.com/v4/letter/d/977dab/32.png) [@DarrenS](https://boards.straightdope.com/u/DarrenS)\
**Post date:** [December 6, 2003, 5:54pm UTC](https://boards.straightdope.com/t/whats-this-unit-of-measurement-mg-1-day-1/217403/2 "2003-12-06T17:54:05Z")

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“per milligram per day”

An exponent of -1 means reciprocal ( x[sup]-1[/sup] = 1 / x )

This is a little weird - I’d expect mg day[sup]-1[/sup] without the (-1) on mg.

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**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\
**Post date:** [December 6, 2003, 5:55pm UTC](https://boards.straightdope.com/t/whats-this-unit-of-measurement-mg-1-day-1/217403/3 "2003-12-06T17:55:29Z")

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You’d read it as “per milligram per day”. The notation x[sup]-1[/sup] is another (and often more convenient) notation for 1/x.

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**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\
**Post date:** [December 6, 2003, 5:58pm UTC](https://boards.straightdope.com/t/whats-this-unit-of-measurement-mg-1-day-1/217403/4 "2003-12-06T17:58:10Z")

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> [@](#):
>
> \*Originally posted by DarrenS \*  
> \*\*This is a little weird - I’d expect mg day[sup]-1[/sup] without the (-1) on mg. \*\*

It’snot so weird. I’ve seen it more often as mg of a drug per kilogram of body weight per day, or X mg kg[sup]-1[/sup] day[sup]-1[/sup].

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**Author:** ![TWDuke](https://avatars.discourse-cdn.com/v4/letter/t/e495f1/32.png) [@TWDuke](https://boards.straightdope.com/u/TWDuke)\
**Post date:** [December 6, 2003, 6:16pm UTC](https://boards.straightdope.com/t/whats-this-unit-of-measurement-mg-1-day-1/217403/5 "2003-12-06T18:16:42Z")

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> [@](#):
>
> \*Originally posted by Q.E.D. \*  
> \*\*It’snot so weird. I’ve seen it more often as mg of a drug per kilogram of body weight per day, or X mg kg[sup]-1[/sup] day[sup]-1[/sup]. \*\*

Ah, it makes sense now! My example should have read “14 mg kg[sup]-1[/sup] day[sup]-1[/sup]”, so no wonder it looked weird. Sorry about the error, and thank you both for your help.