Gallons per Square Foot Calculator

Find gallons per square foot and total volume from an area and a depth.

Feet and inches, metric area and depth, and US or UK gallons.

Updated October 19, 2026

CreatorFrank Zhao

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What this calculator answers

Enter a footprint and a liquid depth, and two numbers come back: the total volume in the gallon unit you choose, and the amount that stands over a single square foot at that depth. The same two numbers work in reverse, which is what makes the tool useful when you already know how much liquid you have.

The idea in one line

G_{\mathrm{US\,gal/ft^2}} = H_{\mathrm{ft}} \times \frac{576}{77} \approx 7.48052\,H_{\mathrm{ft}}

A per-square-foot reading is a depth written as a volume. It depends on how deep the liquid is and on nothing else, so the footprint changes the total but never the rate.

What you put in

A base area, or a length and a width to calculate it, in square feet, square metres, acres and other area units.
A depth, in feet, feet and inches, metres, centimetres and similar length units.
Optionally a known volume, when you are converting an amount you already have.

What you get back

The total liquid volume in the unit you pick: US or UK gallons, litres, cubic feet, fluid ounces and more.
The volume standing over one square foot at that depth, shown in the same volume units.
Whichever box you left empty: the area, the depth, or one side of the rectangle.

Paint, sealer and coating coverage, pond and water-feature top-ups, tank and basin planning, and dosing a liquid across a bed all reduce to the same multiplication: area times depth. The per-square-foot figure is the one to watch when a product datasheet states coverage in gallons per square foot, because that figure is a depth in disguise.

How to use it

Enter the footprint

For a rectangular base, open the collapsed area group and type the length and the width. If you already know the area, type it directly into the area box instead; square feet, square metres, acres and hectares are all accepted.

Enter the depth

Use the average depth of the liquid, not the depth of the container. For a coating, that is the wet film thickness; for a pond, it is water depth above the base.

Read both results

The volume box gives the total amount for the whole footprint. The gallons-per-square-foot box gives the amount over one square foot, which stays the same when the footprint grows at the same depth.

Or work backwards

Type a volume together with the area and the depth is solved; type a volume with the depth and the area is solved; type a per-square-foot rate with the area and the depth and the total follow. The remaining boxes update as you go.

Worked example: a 12 ft by 20 ft pond, 3 ft deep

A = 12 \times 20 = 240\ \mathrm{ft^2}

V = 240 \times 3 = 720\ \mathrm{ft^3}

V_{\mathrm{US\,gal}}

=

720 \times 7.48052

=

5{,}385.97

G_{\mathrm{US\,gal/ft^2}}

=

3 \times 7.48052

=

22.4416

The pond holds about 5,386 US gallons, and every square foot of its surface sits under 22.4416 US gallons. Doubling the footprint to 480 square feet would double the total to about 10,772 US gallons while the per-square-foot figure stays at 22.4416.

Read the per-square-foot figure as a depth in volume form, and the total as that depth spread over the whole footprint. When a supplier quotes coverage per square foot, compare your depth-derived figure with the datasheet instead of comparing total gallons, because the total depends on how large the area happens to be.

The formulas behind the result

Three multiplications cover the whole calculator. Each one is solved in either direction, so any single missing value can be recovered from the others.

A_{\mathrm{ft^2}} = L_{\mathrm{ft}} \times W_{\mathrm{ft}}

V_{\mathrm{ft^3}} = A_{\mathrm{ft^2}} \times H_{\mathrm{ft}}

V_{\mathrm{US\,gal}} = V_{\mathrm{ft^3}} \times \frac{576}{77}

The per-square-foot box is the depth itself, expressed as a volume per unit area: one cubic foot of liquid over one square foot of floor is exactly one foot deep. The calculator therefore shows $\ G = H_{\mathrm{ft}} \times \frac{576}{77}\$ in US gallons, and the same relationship holds in every other volume unit in its menu.

Where 7.48052 comes from

A US gallon is defined as exactly $\ 231\ \mathrm{in^3}\$ and a cubic foot is exactly $\ 1{,}728\ \mathrm{in^3},\$ so one cubic foot holds $\ 1{,}728 \div 231 = \frac{576}{77}\$ US gallons. That ratio is $\ 7.48052\ldots$ and the familiar 7.48052 is its rounded form. Calculating with the exact fraction keeps results consistent when the same depth is read in litres or cubic feet.

Every input is first converted to a common base: lengths to feet, areas to square feet, volumes to cubic feet. That is why switching the unit menu changes the number on screen but never the underlying result.

Solving backwards

Because each relationship is a single multiplication, the reversed forms are simple divisions:

H_{\mathrm{ft}} = \frac{V_{\mathrm{ft^3}}}{A_{\mathrm{ft^2}}}

A_{\mathrm{ft^2}} = \frac{V_{\mathrm{ft^3}}}{H_{\mathrm{ft}}}

W_{\mathrm{ft}} = \frac{A_{\mathrm{ft^2}}}{L_{\mathrm{ft}}}

L_{\mathrm{ft}} = \frac{A_{\mathrm{ft^2}}}{W_{\mathrm{ft}}}

A zero in a denominator has no meaning here, which is why a depth or a side of zero is rejected rather than solved: a footprint with no width and a liquid with no depth both describe nothing to measure.

US gallons, UK gallons and metric units

A UK gallon is larger than a US gallon: $\ 1\ \mathrm{UK\ gal} = 4.54609\ \mathrm{L}\$ against $\ 3.785411784\ \mathrm{L}\$ for the US gallon. A cubic foot therefore holds $\ 6.22884\ \mathrm{UK\ gal}\$ instead of 7.48052 US gallons, so the same depth shows a UK-gallon figure about 17% smaller, the inverse of the $\ 1.20095\$ size ratio. Choosing the wrong gallon type puts a bulk order about 20% out.

If you need the same figure per square metre, note that $\ 1\ \mathrm{ft^2} = 0.09290304\ \mathrm{m^2},\$ so one US gallon per square foot equals $\ 10.7639\ \mathrm{US\,gal/m^2};\$ one foot of depth is exactly $\ 304.8\ \mathrm{L/m^2}.\$ For conversions between US, UK and dry gallons, or for the weight of a volume of water, the gallon calculator covers those units directly.

Assumptions built into the result

The depth is the same everywhere over the footprint.
The footprint is flat and level, so the liquid spreads evenly.
Only the base area counts; vertical walls are not part of the volume.

Worked examples

Estimating a sealer film on a concrete pad

A 1,200 square foot pad needs a 1/16 inch wet film. The depth is entered as 0.0625 inches, which the calculator converts to 0.005208 feet.

G

=

0.005208 \times 7.48052

=

0.038961\ \mathrm{US\,gal/ft^2}

V_{\mathrm{US\,gal}}

=

1{,}200 \times 0.038961

=

46.7532\ \mathrm{US\ gal}

The pad takes 6.25 cubic feet of product, or 46.7532 US gallons. Buy at least 47 gallons and add whatever loss allowance the manufacturer recommends for the surface texture.

Dosing a bed in metric units

A 25 square metre bed is to receive 8 centimetres of liquid feed. Both figures can be entered in their own units; the calculator normalises them.

V

=

25 \times 0.08

=

2\ \mathrm{m^3}

=

2{,}000\ \mathrm{L}

G

=

0.262467\ \mathrm{ft} \times 7.48052

=

1.96339\ \mathrm{US\,gal/ft^2}

That is 2,000 litres, about 528.3 US gallons, or 1.96339 US gallons per square foot (roughly 7.43 litres per square foot). The per-square-foot figure matters if the dosing chart you are following is written in imperial units.

Working back from a container you already have

Suppose you own 20 US gallons of product and want to know how far it stretches across a 100 square foot slab.

G

=

\frac{20}{100}

=

0.2\ \mathrm{US\,gal/ft^2}

H

=

0.2 \times \frac{77}{576}

=

0.026736\ \mathrm{ft}

\approx

0.32\ \mathrm{in}

Enter the 20 gallons and the 100 square feet, and the depth appears as about 0.027 feet, or 0.32 inches. If the product needs a thicker film than that, the container covers less area than the slab.

Practical tips

One inch, one rule of thumb: an inch of depth over a square foot is about $\ 0.6234\ \mathrm{US\ gal}.\$ Multiply by the depth in inches to sanity-check any result before you order.

Average several depth readings. A pond bottom or a slab rarely sits perfectly level, so measure at three or four points, average them, and enter the average.
Use the base footprint, not the wetted surface. Vertical walls do not add volume in this model, so a tank's side area should never be folded into the footprint.
Split irregular shapes. Break an L-shaped or stepped area into rectangles, add their areas, then enter the total as a single area.
Match the precision to the measurement. A depth measured to 0.1 feet carries only about $\ 0.75\ \mathrm{US\,gal/ft^2}\$ of resolution, so treat a long decimal as a calculation result, not as field accuracy.
Check which gallon the label uses. The same volume in UK gallons is a smaller number, and a product priced per imperial gallon contains about a fifth more liquid than the price per US gallon suggests.
Round up for purchases. The arithmetic is exact for the numbers entered; the amount you buy is not. Add the manufacturer's loss allowance for absorption, overspray or uneven ground.

Limits of the estimate

The arithmetic is exact, but it only describes a flat layer of liquid on a level base. These are the boundaries worth knowing before you rely on the result.

•One uniform depth. Sloping bottoms, curved basins and tapered tanks need the volume split into sections or measured directly, because a single depth cannot represent them.
•Base area only. The result does not add freeboard, wall area, pipe volume or the liquid that stays behind in a drain, so it is not a substitute for tank or structural design.
•Displayed rounding. The boxes show a compact, rounded figure rather than every digit of the arithmetic. That rounding is far smaller than the uncertainty in a measured depth or area, so it is rarely what limits a result.
•Gallon definitions differ. US and UK gallons are not interchangeable, and the US dry gallon is not offered here. Confirm which one your supplier, specification or label uses.
•No losses or absorption. Porous surfaces, evaporation, wind and application technique all change how much product a job actually consumes; check the manufacturer's coverage figure for the material before ordering.
•Planning aid, not engineering advice. For tanks, pools, storage or anything holding liquid at depth, verify capacity, loads and local requirements with a qualified professional.

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