How to use this calculator
- Enter or choose dimensions in.
- Enter or choose top length (at water level).
- Enter or choose top width (at water level).
- Enter or choose water depth.
- Enter or choose side slope (horizontal : 1 vertical).
- The pond volume updates as you type. Use Copy result or Share to keep it.
Method
Bottom = Top − 2·z·d; V = d ÷ 6 × (A_top + 4·A_mid + A_bottom)
- d
- water depth
- z
- side slope, horizontal per unit vertical
- A_top, A_bottom
- areas of the water surface and the floor
- A_mid
- area at half depth: (L − z·d)(W − z·d)
Worked example
- Bottom = (30 − 2×2×3) × (20 − 12) = 18 × 8 = 144 m².
- Mid-depth = (30 − 6) × (20 − 6) = 24 × 14 = 336 m²; top = 600 m².
- V = 3 ÷ 6 × (600 + 4×336 + 144) = 0.5 × 2,088 = 1,044 m³.
A 30 × 20 m pond, 3 m deep with 2:1 sides, holds 1,044 m³, about 1.04 million litres.
What the result means
Usable storage is less than the full volume: allow for evaporation, seepage and the dead storage below the pump intake. Measure dimensions at the water line you expect, not at the top of the bank.
Assumptions and limits
- The pond is rectangular with the same slope on all four sides and a flat floor.
- Irregular or round ponds need to be approximated or surveyed.
- Sediment build-up reduces volume over time.
Questions
Why not just multiply length × width × depth?
With sloping sides that overstates the volume. For the example it would give 1,800 m³ instead of 1,044 m³.
Is the prismoidal formula exact here?
Yes. For a straight-sided pit whose cross-section changes linearly with depth, the prismoidal formula gives the exact volume.