The label says 500 gallons. The delivery driver, the rainwater harvester, and the fuel supplier all plan around that number. But a "500-gallon" tank almost never gives you 500 usable gallons, and if the label is missing entirely (a second-hand tank, a farm tank that has been there for decades), you have no number at all. The good news: calculating cylindrical tank volume in gallons takes two measurements and one formula. This guide covers the formula in feet, inches, and metric, a capacity chart for common tank sizes, the weight of a full tank, US versus imperial gallons, horizontal tanks, and the two traps that catch nearly everyone.
The cylindrical tank volume formula
A cylindrical tank is a cylinder standing on end, so its volume is the familiar:
V = π × r² × h
Measure the diameter across the top (inside, if you can), halve it to get the radius r, and measure the height h. Work in feet and the answer comes out in cubic feet. Then apply the single conversion this whole topic hangs on:
1 cubic foot = 7.48052 US gallons
So the full recipe is: gallons = π × r² × h × 7.48052, with r and h in feet.
Worked example. A vertical water tank is 4 feet across and 6 feet tall. Radius = 2 feet.
V = π × 2² × 6 = 75.40 cubic feet
Gallons = 75.40 × 7.48052 = 564 gallons
That is the tank's total (brim-full) capacity. Hold on to that word "total"; it matters below.
If you would rather not do the arithmetic twice, a cylinder volume calculator runs the same formula with the unit conversion built in: enter diameter and height, choose gallons, done. The formula above is all it is doing under the hood.
Tank volume in gallons from inches
Small tanks, drums, and stock tanks are usually measured in inches. The formula is identical, but the conversion changes: a US gallon is exactly 231 cubic inches, so:
Gallons = π × r² × h / 231 (r and h in inches)
Worked example. A tank measures 36 inches across and 48 inches tall. Radius = 18 inches.
V = π × 18² × 48 = 48,858 cubic inches
Gallons = 48,858 / 231 = 211.5 gallons
This also explains a small mystery: a standard "55-gallon drum" (about 22.5 inches across, 33.5 inches tall) computes to 57.7 gallons brim-full. The 55 on the label is the nominal fill, not the geometry. Use whichever unit your tape measure gives you, but never mix them: radius in inches with height in feet is the quietest way to produce a nonsense answer.
If reaching across the top is awkward, wrap a tape around the tank instead and divide by pi: our guide to diameter from circumference walks through that method step by step, including the direct volume formula V = C squared times h divided by 4 pi.
Cylindrical tank capacity chart (vertical tanks, US gallons)
| Diameter | Height | Total capacity |
|---|---|---|
| 2 ft | 3 ft | 70.5 gal |
| 3 ft | 5 ft | 264.4 gal |
| 4 ft | 6 ft | 564 gal |
| 5 ft | 5 ft | 734 gal |
| 6 ft | 4 ft | 846 gal |
| 8 ft | 6 ft | 2,256 gal |
| 10 ft | 8 ft | 4,700 gal |
| 12 ft | 10 ft | 8,460 gal |
Two patterns in the chart are worth memorizing. Doubling the diameter roughly quadruples the capacity (a 4 ft tank holds about four times a 2 ft tank of similar height), because the diameter feeds the formula squared. And doubling the height exactly doubles it. When you are choosing between a wider tank and a taller one, wider wins per foot.
How much does a full tank weigh?
Gallons become pounds faster than people expect. One US gallon of water weighs 8.34 pounds, so the 564-gallon tank above holds 564 × 8.34 = 4,704 pounds of water, about 2.35 tons, before counting the tank itself. Diesel is a little lighter (about 7.1 pounds per gallon), heating oil similar, but the order of magnitude is the same. Before you fill any large tank on a deck, a stand, or a suspended floor, run this multiplication. Structures are rarely rated for the weight of the water people casually plan to store on them.
US gallons vs imperial gallons
Every number in this guide is in US gallons. If you are in the UK, Canada (older references), or anywhere the imperial gallon survives, multiply US gallons by 0.832674: the 564-gallon tank is 469.6 imperial gallons. The imperial gallon is about 20 percent larger, which makes it the single biggest conversion error in this entire topic; we wrote a full breakdown of the two-gallon trap and how it corrupts calculations in our Gallon Bug guide. Check which gallon a label, a supplier, or a second-hand listing means before you plan around its number.
Trap 1: total capacity is not usable capacity
The formula gives the volume to the brim. Real tanks are never used brim-full:
- Headspace. Tanks are filled to about 90 to 95 percent to leave room for expansion and to keep the vent clear. A brim-full 564-gallon tank is realistically a 508 to 536 gallon tank in service.
- The outlet sits above the floor. Anything below the outlet level never drains. On a typical vertical tank that dead zone is another 2 to 5 percent.
Put together, usable capacity is usually about 90 percent of the label. The 500-gallon tank from the introduction delivers roughly 450. Size for the usable number, not the printed one, or your "three days of reserve" quietly becomes two and a half.
Trap 2: the diameter is not the radius
The most expensive slip in this calculation is entering the diameter where the radius belongs. Because the radius is squared, the error squares too: using the 4-foot diameter as the radius in the worked example gives 2,256 gallons instead of 564, an answer exactly four times too big. If a result ever looks surprisingly generous, check this first. Halve the diameter before you do anything else, every time.
Horizontal cylindrical tank volume in gallons
Everything above assumes the tank stands upright. Horizontal cylindrical tanks (fuel bowsers, septic tanks, many farm tanks) follow the same formula for total capacity: π × r² × length × 7.48052, with the length in place of the height. The hard part is a partially filled horizontal tank, where volume does not rise evenly with depth: the first quarter of the depth holds only about a fifth of the volume (19.6 percent), exactly half depth is exactly half full, and the last quarter adds less than the middle quarters. A dipstick reading therefore cannot be converted with a simple percentage. If your tank lies down and you need the volume at a given fill depth, use the dedicated method for horizontal cylinder volume rather than adapting the upright formula and guessing.
Need litres instead?
Multiply US gallons by 3.78541. The 564-gallon tank above is 2,135 litres. Or skip gallons entirely and work in metric from the start: with diameter and height in metres, V = π × r² × h gives cubic metres, and one cubic metre is exactly 1,000 litres. A tank 1 metre across and 1.5 metres tall holds π × 0.5² × 1.5 = 1.178 m³ = 1,178 litres (311.2 US gallons, if you ever need to compare it against an American label). Pick one unit system at the start and stay in it to the end; most wrong answers in this topic are born mid-calculation, in a half-finished conversion.
Sizing a tank backwards
The same formula runs in reverse when you are buying rather than measuring. Suppose you need 1,000 usable gallons. Divide by 0.9 to undo the usable-capacity discount: you need a tank with a total capacity of about 1,111 gallons. Fix the diameter your space allows, say 5 feet. Its cross-section is π × 2.5² = 19.63 square feet, so every foot of height adds 19.63 cubic feet, or 146.9 gallons. Required height: 1,111 / 146.9 ≈ 7.6 feet. Three numbers (target, diameter, gallons per foot) and you can spec a tank on the phone with a supplier instead of guessing from a brochure photo.
Frequently asked questions
How many gallons does a 4 ft by 6 ft cylindrical tank hold?
564 US gallons brim-full, and about 508 gallons usable once headspace and the outlet dead zone are allowed for.
What is the formula for cylindrical tank volume in gallons?
Gallons = π × r² × h × 7.48052 with radius and height in feet, or π × r² × h / 231 with both in inches. Always halve the diameter to get the radius first.
How many gallons is a 10-foot diameter tank?
It depends on the height: at 8 feet tall, 4,700 US gallons. Every additional foot of height adds about 587 gallons at that diameter.
Is a horizontal tank half full at half depth?
Yes, exactly half by volume at exactly half depth. But a quarter depth is only about 19.6 percent of the volume, not 25 percent, which is why dipstick charts for horizontal tanks are never evenly spaced.
How much does 500 gallons of water weigh?
500 × 8.34 = 4,170 pounds, just over 2 tons, not counting the tank.
The 60-second version
- Measure the inside diameter and the height.
- Halve the diameter to get the radius. (Do not skip this.)
- Gallons = π × r² × h × 7.48052 (feet) or / 231 (inches).
- Take 90 percent of the result as your usable capacity.
- If the tank lies on its side, use the horizontal-tank method for partial fills.
A round tank hides nothing once you know the recipe: two measurements, one squared radius, one conversion. The label's job is marketing. The formula's job is the truth.
