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Tank Volume Calculator | Compute Tank Gallons From Dimensions

Calculate the capacity of rectangular and cylindrical tanks in gallons and litres from their interior dimensions, with partial-fill support. Perfect for aquariums, fuel storage, water tanks and chemical vessels — the volume math is shown step by step.

Last updated: July 2026 · Free · No sign-up required

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Results

Enter values above and click Calculate to see your result instantly.

How the Tank Calculator works

Every result on this page comes from a real formula — Gallons = L(ft) × W(ft) × H(ft) × 7.48052 — computed live in your browser the moment you press the button.

For context, the sections beneath the calculator include typical values, a worked example you can recompute by hand, and an FAQ covering the practical details of tank volume calculator.

From our guides

Quick reference

Tank (ft)Cubic feetUS gallons
4 × 2 × 216119.7
6 × 3 × 354404.0
8 × 4 × 4128957.5
12 × 6 × 42882,154.4

info Tank Calculator

Free math calculator — enter your numbers and get an instant, accurate result.

info Private by design

Everything runs locally in your browser. No uploads, no accounts, no tracking.

info Works everywhere

Fully responsive and mobile-friendly — calculate on any device, any time.

info Educational

Includes the formula and step-by-step explanation so you understand the math, not just the answer.

Interpretation guide

OutputUse
Cubic feetCross-check manufacturer specs
US gallonsPump sizing, water delivery, chemical dosing
× 8.34 lb/galWeight of the water for structural checks
÷ heightGallons per foot for partial-fill readings

lightbulb Worked example

Let's say you are using the Tank Calculator. Calculates tank volume using Gallons = L(ft) × W(ft) × H(ft) × 7.48052, live in your browser. Enter the values that match your situation into the input fields and press calculate — using realistic numbers makes the result directly useful for you.

Result: The calculator instantly applies the formula Gallons = L(ft) × W(ft) × H(ft) × 7.48052 and returns the result with appropriate precision.

What this means: Read the result in the context of what you are measuring. The step-by-step breakdown lets you confirm the math and understand which input most affects the outcome.

Formula & methodology

Formula: Gallons = L(ft) × W(ft) × H(ft) × 7.48052

How tank capacity is calculated

The tool applies Gallons = L × W × H (in feet) × 7.48052 for rectangular tanks, and the cylinder formula π × r² × length for round tanks, with partial-fill support for liquid depth. The 7.48052 factor converts cubic feet to US gallons.

Example: a 5 ft × 3 ft × 2 ft tank holds 5 × 3 × 2 × 7.48 = 224.4 US gallons; a 2 ft diameter, 4 ft cylinder holds about 94 gallons.

Reading the result: use interior dimensions for true capacity, since wall thickness reduces the volume — outside measurements overstate by 5–10%. For fuel tanks, the usable capacity is also less than the geometric volume because of the pickup, vent, and expansion allowance. The partial-fill mode converts a dipstick reading into remaining gallons.

Authoritative source: Wolfram MathWorld

Frequently asked questions

How do I calculate tank volume in gallons?
Enter the required values into the input fields and press the calculate button. The tool applies the formula Gallons = L(ft) × W(ft) × H(ft) × 7.48052 and returns the result with a short explanation of each step, so you can check the working yourself.
How many gallons in one cubic foot?
The Tank Calculator is free, private, and accurate: it runs entirely in your browser (no uploads, no accounts), applies the standard calculation, and explains each step so you can verify the result. Calculates tank volume using Gallons = L(ft) × W(ft) × H(ft) × 7.48052, live in your browser. There is no limit on usage, and it works on any device.
Should I measure inside or outside?
Measure the inside dimensions for capacity — the volume that actually holds liquid — because wall thickness reduces the interior space. Outside dimensions overstate capacity by the wall thickness on every side, which matters more for small tanks. For rectangular tanks multiply inside length, width, and height; for round tanks use the inside diameter and length.
How much does a full tank weigh?
Multiply the volume by the liquid density: water weighs about 8.34 lb per US gallon (1,000 kg per m³), while fuel, oil, and chemicals weigh less or more. Add the tank's empty weight to get the total. A 100-gallon water tank holds roughly 834 lb of water, which is why tank supports and floors need structural consideration before filling.
Does this work for round tanks?
Yes — the calculator includes cylindrical tanks, using the formula π × r² × length for the volume, with the radius being half the inside diameter. Horizontal cylinders need the segment-area formula for partially filled states, which the tool handles when you enter a fill depth. Vertical cylindrical tanks are straightforward: base area times liquid height.
How many gallons are in a round (cylindrical) tank?
Volume = π × radius² × height, then × 7.48 to convert cubic feet to US gallons. A 4 ft diameter tank standing 5 ft tall holds π × 2² × 5 = 62.8 cu ft = 470 gallons. For a horizontal cylinder that's only partly full, the partial-fill math is nonlinear — half the depth is more than half the volume only at exactly 50%.
What size well pressure tank do I need?
Size by drawdown — the water actually delivered between pump cycles — not the tank shell size. Rule of thumb: 1–2 gallons of drawdown per gallon-per-minute of pump flow, so a 10 GPM pump wants 10–20 gallons of drawdown, which is roughly a 30–44 gallon diaphragm tank (drawdown is only ~30–35% of shell volume at standard 30/50 pressure switch settings).
What does the Tank Calculator do?
Calculates tank volume using Gallons = L(ft) × W(ft) × H(ft) × 7.48052, live in your browser. The calculator takes your inputs, applies the standard calculation, and returns a clear result so you can make an informed decision without doing the math by hand.
What formula does the Tank Calculator use?
It calculates rectangular-tank capacity as Gallons = L(ft) × W(ft) × H(ft) × 7.48052, where 7.48052 is the number of US gallons in one cubic foot. Enter the interior dimensions in feet — or inches with the unit toggle — and the tool returns capacity in gallons and litres, so you can size a fish tank, fuel tank, or storage vessel correctly.
Is the Tank Calculator free and private?
Yes — the Tank Calculator. There is no sign-up, no paywall, no trial, and no limit on how many calculations you can run. CalcaTools covers its costs with non-intrusive display advertising, so the calculator itself never asks for payment or restricts any feature.
How accurate is the Tank Calculator?
The Tank Calculator applies the standard formula: Gallons = L(ft) × W(ft) × H(ft) × 7.48052. It is tested against published worked examples before launch and uses native double-precision arithmetic, so results are accurate to typical precision for the values you enter — with no intermediate rounding that could distort the answer.

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