Farm Dam Volume Calculator
Estimate the water capacity of a dug or turkey-nest farm dam shaped like a truncated pyramid. Enter the top and bottom length and width plus the depth, and get the volume in cubic metres, kilolitres and megalitres.
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Measure the top length and width at full-supply level (the spillway/overflow line).
- Measure the bottom length and width and the vertical depth from floor to full-supply level.
- Read the estimated volume in m³, kL and ML; treat it as a geometric upper bound before seepage and evaporation.
How it works
The dam is modelled as a frustum (truncated pyramid). Its volume by the prismoidal rule is V = depth ÷ 3 × (A_top + A_bot + √(A_top × A_bot)), where A_top = top length × top width and A_bot = bottom length × bottom width. That gives cubic metres, which convert directly to kilolitres (1 kL = 1 m³) and megalitres (1 ML = 1000 m³).
Worked example
Worked example. A dam 30 m × 20 m at the top, 20 m × 10 m at the bottom and 4 m deep: A_top = 600 m², A_bot = 200 m², V = 4 ÷ 3 × (600 + 200 + √(600 × 200)) = 1.3333 × 1146.41 = 1,528.55 m³ ≈ 1.529 ML.
Common mistakes
- Measuring the top above the spillway line, which overstates the usable capacity.
- Ignoring battered sides and an uneven floor — real dams hold less than the clean geometric shape.
- Forgetting seepage and evaporation, which reduce the water actually available over a season.
Frequently asked questions
Why use the prismoidal formula instead of averaging the areas?
The prismoidal (frustum) formula V = h/3 × (A_top + A_bot + √(A_top·A_bot)) is the exact volume of a truncated pyramid; a simple average of top and bottom areas overestimates a tapering dam.
Is this my usable water?
No — it is a full-supply geometric estimate. Subtract dead storage below the outlet, plus ongoing seepage and evaporation losses, for the water you can actually draw.
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Tip: Enter any known values to calculate the remaining results.
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