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One acre covered with water 1 foot deep holds about 325,851 US gallons of water.
This volume is called an acre-foot. It is a common unit for measuring large amounts of water used for ponds, irrigation, reservoirs, and water storage.
How Many Gallons Are in 1 Acre-Foot?
The calculation starts with the size of one acre:
- 1 acre = 43,560 square feet
- Water depth = 1 foot
- Volume = 43,560 cubic feet
One cubic foot holds about 7.4805 US gallons.
So:
43,560 × 1 × 7.4805 = about 325,851 gallons
That means:
1 acre × 1 foot deep = about 325,851 gallons
This assumes the entire acre is covered evenly to a full depth of 1 foot.
Acre-Foot Water Volume Table
| Water Depth Over 1 Acre | Approximate Gallons |
|---|---|
| 1 inch | 27,154 gallons |
| 3 inches | 81,463 gallons |
| 6 inches | 162,926 gallons |
| 9 inches | 244,389 gallons |
| 12 inches / 1 foot | 325,851 gallons |
| 2 feet | 651,703 gallons |
| 3 feet | 977,554 gallons |
These numbers describe water volume. They do not account for water soaking into soil, flowing away, evaporating, or filling low spots first.
The Formula for Any Water Depth
You can estimate the gallons needed to cover an acre using:
Gallons = Acres × Depth in feet × 325,851
For example, covering 2 acres with water 1 foot deep would require:
2 × 1 × 325,851 = about 651,703 gallons
If the desired depth is 6 inches, convert it to feet first:
6 inches ÷ 12 = 0.5 foot
Then:
1 acre × 0.5 × 325,851 = about 162,926 gallons
What If You Know the Depth in Inches?
For shallow water depths, another useful rule is:
1 inch of water over 1 acre = about 27,154 gallons
You can therefore use:
Gallons = Acres × Depth in inches × 27,154
For example, 4 inches of water over one acre would be:
1 × 4 × 27,154 = about 108,616 gallons
This formula is useful for irrigation and rainfall calculations because water depth is often measured in inches rather than feet.
Does It Really Take 325,851 Gallons to Flood an Acre?
It takes about 325,851 gallons to create a theoretical average depth of 1 foot across one level acre.
Real land is usually uneven. Water moves toward low areas instead of forming a perfectly level layer everywhere. Some water may also:
- soak into the ground
- drain through soil
- run off the property
- enter ditches or drains
- fill depressions
- evaporate
Because of these losses and variations, the amount of water needed to produce an actual 1-foot-deep flooded area can differ greatly from the simple volume calculation.
If the goal is pond filling, irrigation, or property drainage planning, the shape and elevation of the land matter just as much as the acreage.
How Much Does an Acre-Foot of Water Weigh?
Consider water volume from two inches of rain on an acre to assess catchment details that handle both rainfall and debris.
A gallon of water weighs about 8.34 pounds under ordinary conditions.
Using that figure:
325,851 gallons × 8.34 pounds = about 2.72 million pounds
That is roughly 1,360 tons of water.
This large weight is one reason stored water requires properly designed tanks, foundations, pond structures, and retaining systems. Large water volumes should not be placed against buildings or contained by improvised walls.
How Does This Compare With Rainfall?
One foot of accumulated rain would also equal about 325,851 gallons per acre if every drop stayed on that acre.
More commonly:
1 inch of rain on 1 acre = about 27,154 gallons
However, rainfall does not normally remain as a uniform layer. Some becomes runoff, some enters the soil, and some is intercepted by plants or other surfaces.
For rainwater harvesting, you would normally calculate collection from the actual roof or catchment area rather than from the total land area.
Using This Number for Irrigation
You usually do not need anything close to an acre-foot of water for a single irrigation cycle.
For example, applying 1 inch of water across an entire acre represents about 27,154 gallons. Even then, the amount that must actually be supplied depends on rainfall, soil, plant needs, irrigation efficiency, and how much of the acre is being watered.
Avoid treating a fixed water depth as a universal irrigation target. Different crops, soils, climates, and seasons require different watering plans.
Using This Number for a Pond or Cistern
The acre-foot figure can also help with rough pond-volume calculations.
A pond covering one acre with an average depth of 1 foot contains about 325,851 gallons. If its average depth were 5 feet, its approximate volume would be:
325,851 × 5 = about 1,629,257 gallons
The important word is average. A pond that is 5 feet deep at its deepest point does not necessarily have an average depth of 5 feet.
Cisterns and tanks are normally sized by their actual internal dimensions or rated capacity instead.
Frequently Asked Questions
How many gallons does it take to cover 1 acre with 1 foot of water?
About 325,851 US gallons. This amount of water is equal to one acre-foot.
How many gallons cover 1 acre 1 inch deep?
About 27,154 gallons cover one acre to a depth of 1 inch.
How many gallons cover 1 acre 6 inches deep?
About 162,926 gallons. Six inches is half a foot, so it is half of one acre-foot.
How many cubic feet are in an acre-foot?
One acre-foot contains 43,560 cubic feet of water.
How many gallons are in 2 acre-feet?
Two acre-feet contain about 651,703 gallons.
Does soil absorption change how much water is needed to flood an acre?
Yes. The 325,851-gallon figure describes the volume of a uniform 1-foot layer. In real conditions, soil infiltration, runoff, drainage, and uneven ground can change how much water is needed.
Is an acre-foot the same as one foot of rain on an acre?
In terms of total water volume, yes. One foot of water spread evenly across one acre equals one acre-foot, or about 325,851 gallons. Actual rainfall will usually partly infiltrate, run off, or evaporate rather than remain as a one-foot layer.
