Tank Volume Calculator
Get tank capacity in gallons and cubic feet for vertical cylinders, horizontal cylinders, and rectangular tanks — and, just as usefully, the volume at any liquid level measured from the bottom. Horizontal tanks are the case generic calculators fumble: volume is not proportional to depth, so this tool runs the circular-segment math and reports fill percent and remaining headspace (ullage) alongside. Enter dimensions in inches; leave the level blank for brim-full capacity. Free, no login.
Tank
All dimensions in inches. Blank level → full (brim) capacity.
Capacity
Enter dimensions to calculate
How this calculator works
This is pure geometry — there is no code table behind it, and the page cites none. Full capacity is the solid volume: L × W × H for a rectangle, π r² × h for a cylinder, converted at 231 in³ per US gallon. Partial fills in vertical cylinders and rectangles scale linearly with depth. A horizontal cylinder does not — the liquid cross-section is a circular segment:
A = r² × acos((r−h)/r) − (r−h) × √(2rh − h²) · V = A × length
where h is the liquid depth from the bottom. Near the bottom and top of a horizontal tank, an inch of level change moves far fewer gallons than an inch at the middle — which is why a stick reading at 25% depth is not 25% of capacity (it is closer to 20%). The calculator handles all three shapes with the same optional level input and shows fill percent and ullage whenever a level is entered.
Worked example
A horizontal cylindrical tank, 48" diameter × 96" long, gauged at 24" of liquid (half depth):
- Full capacity: π × 24² × 96 = 173,718 in³ ÷ 231 = 752.02 gal.
- Segment at h = 24 (= r): A = 24² × acos(0) − 0 = 576 × π/2 = 904.78 in² — exactly half the circle, as it should be at half depth.
- Volume at level: 904.78 × 96 = 86,859 in³ ÷ 231 = 376.01 gal (50.265 ft³), a 50.0% fill with 376.01 gal of ullage.
Enter the same tank above (Cylinder — Horizontal, 48 × 96, level 24) to reproduce it. Try level 12 to see the segment effect: quarter depth holds 147.02 gal — 19.6% of capacity, not 25%.
Reference: gallons per inch of depth (vertical cylinders)
For vertical tanks the depth-to-gallons relationship is linear, so one number per diameter answers most field questions — computed by the same engine as the calculator.
| Tank diameter | Gallons per inch |
|---|---|
| 12" | 0.49 |
| 18" | 1.10 |
| 24" | 1.96 |
| 30" | 3.06 |
| 36" | 4.41 |
| 42" | 6.00 |
| 48" | 7.83 |
| 60" | 12.24 |
Frequently asked questions
How do I calculate how many gallons a tank holds?
Volume in cubic inches divided by 231 gives US gallons. For a rectangular tank that is length × width × height; for a cylinder it is π × radius² × height (or length, for a tank lying on its side). This calculator takes all dimensions in inches and returns gallons and cubic feet — one cubic foot is 7.48 gallons.
How do I figure the gallons at a partial fill level?
Enter the liquid level measured up from the bottom of the tank, and the calculator returns the volume at that level along with full capacity, percent fill, and the remaining headspace (ullage). Vertical cylinders and rectangular tanks are linear — half the depth is half the volume. Horizontal cylinders are not, so the tool switches to circular-segment math for them.
Why is a horizontal tank not at 25% volume when the stick reads a quarter depth?
Because the cross-section is a circle, the gallons per inch of depth change as the level rises — thin slices near the bottom and top of the shell hold far less than slices through the middle. At 25% of the diameter a horizontal cylinder holds only about 20% of its capacity, and only the half-depth mark corresponds to exactly 50%. This is why horizontal tanks are gauged with strapping charts; the segment calculation here is the same math a strapping chart tabulates.
Is full capacity the same as usable capacity?
No. This calculator returns geometric brim-full capacity; working volume is always less — freeboard at the top, dead volume below the outlet, and in pumped systems only the draw between pump-on and pump-off (or high and low alarm) levels is actually usable. Septic tanks and grease interceptors are also rated on liquid operating depth, not brim volume. Derate before committing a tank size on a bid.
How much does a full water tank weigh?
Water runs about 8.34 lb per gallon, so a 500 gallon tank carries over 4,000 lb of water before the weight of the tank itself. Check the structure, pad, or stand rating before setting or hanging any sizable tank, and remember other liquids differ — many chemical solutions run heavier than water.
Method: solid geometry at 231 in³ per US gallon and 1,728 in³ per cubic foot — box and cylinder volumes for full capacity, the circular-segment area for partly filled horizontal cylinders. This is a geometric figure with no code citation: it is the brim-full interior volume, so real working capacity is less after freeboard, inlet/outlet positions, fittings and internal structures, head shapes on ASME-style tanks (dished ends add volume this right-cylinder model omits), and the usable draw between control levels. For rated equipment — water heaters, well tanks, interceptors — the manufacturer’s listed capacity governs.
Pair with the Pipe Volume & Capacity Calculator to add the piping to a system fill, the GPM Flow Rate Calculator for fill and drain-down times, or the grease interceptor sizing chart where the vessel is an interceptor.