Tank & Trench Calculator
Answer: an 8 ft diameter tank, 4 ft deep, holds about 201 cu ft — 7.45 cubic yards, or about 1,504 US gallons. A 40 × 2 × 3 ft trench takes 8.89 cubic yards net.
Two calculators on one page: cylindrical tanks (volume in cubic yards, cubic feet, cubic meters and US gallons) and rectangular trenches (volume plus bag counts). Both use the same yields and waste rules as the rest of this site.
Cylindrical tank volume
Rectangular trench volume
How the math works
Cylinder: volume = π × radius² × height, where the radius is half the diameter. Trench: volume = length × width × depth. Both give cubic feet first, then convert:
- 1 cubic yard = 27 cubic feet.
- 1 cubic meter = 35.3147 cubic feet.
- 1 cubic foot = 7.48052 US gallons.
Bag counts use the order volume (after waste) and QUIKRETE published yields, the same as the site's main calculator.
| Worked example | Calculation | Result |
|---|---|---|
| Tank: 8 ft diameter, 4 ft deep | π × 4² × 4 | 201.1 cu ft = 7.45 yd³ = 1,504 gal |
| Tank order volume, 10% waste | 7.45 × 1.10 | 8.19 yd³ (369 × 80 lb bags) |
| Trench: 40 × 2 × 3 ft | 40 × 2 × 3 | 240.0 cu ft = 8.89 yd³ |
| Trench order volume, 10% waste | 8.89 × 1.10 | 9.78 yd³ (440 × 80 lb bags) |
Inside dimensions, and why they matter
For a tank, enter inside dimensions: the diameter and depth of the space the concrete or liquid fills. The calculator does not deduct wall thickness. For a tank shell or lining, measure the space the concrete fills, not the outside of the tank.
For a trench, measure the dug width at several points and use the widest — a hand-dug trench bulges, and the extra width is concrete you will pour. That is why a hand-dug trench needs 15–20% waste instead of the 10% default used for straight, formed work.
How to use these calculators
- For a tank: measure the inside diameter and the inside depth. For a trench: measure the dug length, width and depth.
- Choose a waste allowance — 10% for formed work, 15–20% for a hand-dug trench.
- Read the volume in cubic yards, cubic feet and cubic meters, the gallons for a tank, and the bag counts.
Frequently asked questions
How many gallons does an 8-foot round tank hold?
At 4 ft deep, about 201 cubic feet, or about 1,504 US gallons, before waste. Enter inside dimensions — the calculator does not deduct wall thickness.
How much concrete do I need for a 40-foot trench?
At 2 ft wide and 3 ft deep, the net volume is 8.89 cubic yards. With 10% waste, order 9.78 cubic yards — 440 bags of 80 lb mix from the order volume.
Do I enter inside or outside dimensions for a tank?
Inside dimensions. The calculator does not deduct wall thickness, so measure the space the concrete fills.
What waste allowance should I use for a hand-dug trench?
Use 15–20% for a hand-dug trench, where the sides bulge, instead of the 10% default. Measure the dug width at several points and use the widest.
How do you calculate a concrete cylinder?
Volume = π × radius² × height, with the radius half the diameter. A 12 in diameter column 4 ft high is π × 0.5² × 4 = 3.14 cu ft, or 0.12 cubic yards. Use the column calculator for piers and sonotubes, or the tank section above for tanks.
Can I use this calculator for flowable fill?
Yes, for the volume: flowable fill (also called controlled low-strength material, or CLSM) is ordered by the cubic yard like concrete, and the trench volume here — length × width × depth — is the same calculation. Enter your trench dimensions above. Confirm the mix strength and whether it must stay excavatable with your supplier: flowable fill is much weaker than structural concrete by design.