# Talk:Acre-foot

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## Precision

When specifying any quantity via numbers for the purpose of explaining the idea to humans, it is rarely necessary or even helpful to use numbers of more than three or sometimes four decimal places in order to fasten upon the needed degree of precision. Thus, from this artcle,

"Definition

"It is defined by the volume of water necessary to cover one acre of surface area to a depth of one foot. It is equal to exactly 43,560 cubic feet, or 325,853.382 U.S. gallons, or 1233.489238 cubic meters".

(Unfortunately, it appears that the popular and incorrect calculation of 325,851 or 325,853 gallons per acre foot is being perpetuated throughout the known world! All this because of the lack in ability to do this simple calculation. I will illustrate here: 1 acre (1 foot thick) x # of gallons in a cubic foot. This is, simply 43560 (Cubic feet) x 7.48 gallons / cubic foot = 325,828.8 gallons - rounded to 325,829 gallons per acre foot (do it on a calculator!). I find it odd that this number is wrong everywhere I look! Am I missing something? Well, Yes I am. I have discovered that the gallons per cubic foot of water has been rounded (one has to watch out for those rounding issues) and needs to be at least 7.4805 gallons per cubic foot to obtain the globally accepted number 325851 gallons per acre foot - I am now a humbled Water District Engineering Technician...)

For general description purposes, these could also be written as 43.6K cubic feet, 326K gal, 123.5K cubic meters, respectively.

The heading is "Definition" so what makes you think its a "general description"? Roger 11:24, 25 August 2007 (UTC)
Exactly: highly precise conversions are perfectly appropriate in a definition and exact values are better still. To avoid your rounding errors note that a US gallon is 231 cubic inches and a cubic foot is 123 cubic inches so an acre foot is 660 × 66 × 1728 ÷ 231 or ​325,851 37 US gallons exactly. I have also calculated an exact cubic-metre conversion which I've added into the article.
So we have the following conversions. 1 international acre foot ≡ 43,560 international cubic feet = 1233.48183754752 m3 (exactly) = ​325,851 37 US gallons (exactly) ≈ 271,328.072596 imp gal. I have also added a definition for the US survey version (as noted below).
This replaces the old conversions of "325,851.4 U.S. gallons/271,328.0 imperial gallons/1,233.5 kL/1,233.5 /1.23 ML". You won't see this in the page history (as of today) since {{`convert|325851.4|U.S.gal|impgal|lk=on|disp=/`}} is currently not working (as the template is undergoing an overhaul).
Note the incorrect 271,328.0 imp gal figure (as opposed to 271,328.1 imp gal rounded to the nearest 0.1 imp gal). Don't blame {{convert}}. Feed a rounded figure into the template and you've got to be careful of the rounding in the output.
Note also that I haven't bothered with kilolitres and megalitres. Click on the link to Cubic metre and you'll find that it equals one kilolitre or 0.001 megalitres. Here's not the place to go waffling about the metric system. JIMp talk·cont 07:51, 14 August 2011 (UTC)

## Application

"So Acre feet is specifically used to define a yearly usage of water. It is not a appropriate unit to define a monthly consumption?" —Preceding unsigned comment added by 206.55.149.108 (talk) 22:22, 24 January 2008 (UTC)

Not necessarily. It is a unit of volume. I suppose baking recipes could measure flour and milk in acre-feet, but that's obviously silly. Acre-feet per year is among the most common units for water consumption. I don't recall seeing it used in any other context. —EncMstr 23:02, 24 January 2008 (UTC)
I have never heard of it being used to measure the water consumption of a family home. It comes from irrigation. If you are going to grow rice in a dry area, you need one acre-foot of water to flood a one acre rice field one foot deep with water. Or the equivalent of 12 inches of rainfall.Eregli bob (talk) 14:15, 23 August 2013 (UTC)
If you are growing rice in a wet area, it still takes an acre-foot of water to flood a 1 acre field to a depth of 1 foot. That's just the definition of the unit. Lieutenant pepper (talk) 16:41, 6 March 2014 (UTC)

## survey foot

In the US is this defined in terms of the survey foot (and equivalent acre) or the international foot? The difference would be 7.4 L (1.96 Gallons) --Random832 (contribs) 23:27, 19 June 2008 (UTC)

It would probably be best to give both definitions. One US survey acre foot would thus be 43,560 cubic US survey feet. Since one US survey foot is ​12003937 metres this gives an acre foot of approximately 1233.4892384681 cubic metres. JIMp talk·cont 06:25, 14 August 2011 (UTC)

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## Exact values

1 international foot is exactly 0.3048 m, therefore 1 int ft³ = exactly 0.028316846592 m³. Therefore 1 int acre-foot = exactly 1233.48183754752 m³

1 US gal = 231 in³, therefore 1 acre-foot = exactly 325851 3/7 US gal. Wassermaus (talk) 13:35, 4 May 2018 (UTC)