Dome top tank volume calculator
A dome top tank holds the volume of its straight wall plus the volume of its dome. The wall holds π × r² × wall height. The dome holds 2/3 × π × r² × dome height. Divide cubic inches by 231 for US gallons. The dome holds less per inch than the wall. A 64 in tank holds 13.9 gal per inch in the wall, but only 1.5 gal in the top inch of a 9 in dome. Nothing you enter leaves your browser.
Exact segment geometry — see methodology. Dished-head propane math is documented there too.
What to measure
How to measure a dome top tank
- Wrap a tape around the tank at the widest point. Divide the result by 3.1416 to get the diameter.
- Find the top of the straight wall. It is the line where the dome or the cone starts. On a poly tank, it is often at the top rib or at a seam.
- Measure the height of the straight wall from the floor to that line.
- Measure the overall height to the top of the dome. Do not include the lid.
- Subtract the wall height from the overall height. The result is the dome height.
- Enter the diameter, the wall height and the dome height in the calculator.
- Measure the liquid depth with a stick through the lid, and enter it as the fill level.
Outside sizes give a result that is a few percent high, because of the wall. For a poly tank, subtract two wall thicknesses from the diameter if you know the wall thickness.
Worked example
The example uses the outside sizes of a Norwesco 1000 gal tank from its drawing. By these sizes it holds 1,027.1 gal, which is 2.7% more than its rating. The table shows how the gallons per inch fall in the dome.
| Liquid depth | US gallons | % of full |
|---|---|---|
| 12 in | 167.1 | 16.3% |
| 24 in | 334.2 | 32.5% |
| 36 in | 501.3 | 48.8% |
| 48 in | 668.5 | 65.1% |
| 60 in | 835.6 | 81.4% |
| 67.75 in (top of wall) | 943.5 | 91.9% |
| 70 in | 974.2 | 94.9% |
| 72 in | 998.3 | 97.2% |
| 74 in | 1,016.6 | 99.0% |
| 76 in | 1,026.2 | 99.9% |
| 76.75 in (top) | 1,027.1 | 100.0% |
Above 67¾ in, the depth increases by 9 in, but the volume increases by only 83.6 gal. Do not use a straight-wall gallons-per-inch figure in the dome.
How this is calculated
V = πr²Hcyl + (2/3)πr²a, a = dome heightV = πr²Hcyl + πr²(y − y³/3a²), y = h − HcylV = πr²Hcyl + (πr²Hcone/3)(1 − t³), t = 1 − (h − Hcyl)/HconeThe dome is modelled as a half-spheroid that meets the wall smoothly. Real poly tank domes are close to this shape, but ribs and a flat lid area change the top few inches. The engine is checked against numerical integration, like every other shape on this site.
Charts for real tanks
These pages use this model with each maker's drawing. The values are scaled to the rated capacity. Norwesco 500 · Norwesco 1000 · Norwesco 1550 · Norwesco 2500 · Snyder 1000 · Norwesco 305 (sloped shoulder). Not sure which tank you have? Use find my tank.
FAQ
TVC-120/FAQHow do I calculate the volume of a dome top tank?
Add the straight wall and the dome. The wall is π × r² × wall height. The dome is 2/3 × π × r² × dome height. Divide cubic inches by 231 for US gallons. A 64 in tank with a 67.75 in wall and a 9 in dome holds 1,027.1 gal by its outside sizes.
How do I measure the dome height?
Measure the overall height to the top of the dome, not to the top of the lid. Measure the height of the straight wall. Subtract the wall height from the overall height.
Why does the top of the tank hold less per inch?
The dome narrows as it rises. In the example tank, one inch of the wall holds 13.9 gal, but the top inch of the dome holds only 1.5 gal.
Does the calculator do cone tops too?
Yes. Select Cone Top and enter the cone height. A cone top narrows to a point, so it holds one third of a cylinder of the same height.
Sources and standards
- Methodology — partial-fill formulas and their tested errortankvolumecalc.comhttps://tankvolumecalc.com/methodology/
- 1000 GALLON VERTICAL TANK (Norwesco drawing; file '1000 Gallon X 64 Vertical Tank.pdf')Norwesco, Inc. — manufacturerhttps://www.norwesco.com/_site_components/uploads/pdfs/Vertical/1000%20Gallon%20X%2064%20Vertical%20Tank.pdf
Every linked source above was retrieved and checked on 2026-11-01.