Sump Capacity Calculator
A sump (settling or dewatering pit) is treated as a rectangular box, so its capacity is simply length × width × depth in cubic metres.
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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
- Enter the sump's plan length and width in metres, then the effective (usable) working depth — the depth between the pump cut-off level and the high-water alarm level, not the full excavated depth.
- Optionally enter a flow rate in litres per second: use the expected inflow to see how long the empty sump takes to fill, or the pump-out rate to see how long a full sump takes to clear.
- Read the capacity in m³, kilolitres and litres, and the fill/empty time. Compare the time against your shift dewatering cycle and pump duty to confirm the sump is big enough.
How it works
A sump (settling or dewatering pit) is treated as a rectangular box, so its capacity is simply length × width × depth in cubic metres. Because 1 m³ of water equals exactly 1 kilolitre (1,000 litres), the same number reads as both m³ and kL, and multiplying by 1,000 gives litres. Only the effective depth between the pump start/stop levels represents usable storage, so that is the depth you should enter.
When a flow rate is supplied, fill or empty time = capacity ÷ flow. Working in litres and litres-per-second keeps the units consistent: total litres divided by litres per second gives seconds, which the tool also converts to minutes and hours. The same arithmetic answers two questions — with an inflow rate it estimates how long an empty sump takes to fill; with a pump-out rate it estimates how long a full sump takes to clear.
Worked example
A 6 m × 3 m × 2 m underground sump pumped at 20 L/s. Capacity = 6 × 3 × 2 = 36 m³ = 36 kL = 36,000 L. At a 20 L/s pump-out rate, the time to empty a full sump = 36,000 ÷ 20 = 1,800 s = 30 min. Sizing the sump so the pumps can clear it comfortably within a shift's peak inflow keeps water off the working floor.
Common mistakes
- Using the full excavated depth instead of the effective working depth. Storage is only the volume between the pump cut-off and high-water levels — the dead water below the pump intake and any freeboard above the alarm level don't count.
- Mixing flow units. This tool expects the flow rate in litres per second (L/s). If your pump is rated in L/min or m³/h, convert first (L/min ÷ 60 = L/s; m³/h × 1000 ÷ 3600 = L/s).
- Treating an irregular or tapered sump as a perfect box. Sloped or stepped sides hold less than length × width × depth, so this prismatic figure is an upper bound — reduce it for real geometry.
Frequently asked questions
Is a cubic metre the same as a kilolitre for a sump?
Yes. One cubic metre of water is exactly 1,000 litres, which is 1 kilolitre. So a sump capacity of 36 m³ is 36 kL or 36,000 L — the tool shows all three so you can match whatever unit your dewatering plan uses.
What depth should I enter?
Use the effective (usable) depth — the vertical distance between the pump switch-off level and the high-water alarm level. That is the water the pumps actually move each cycle. Entering the full pit depth overstates usable storage because the water below the pump intake and the freeboard above the alarm are not available.
Does the fill/empty time account for continuing inflow while pumping?
No. The time is a simple capacity ÷ flow estimate for a single rate. In practice groundwater keeps flowing in while you pump out, so a real dewatering pump must be sized above the peak inflow. Treat the empty-time figure as a lower bound and confirm pump duty against measured inflow.
Can I use this for a surface settling pond or turkey's nest dam?
For a roughly rectangular pond with near-vertical sides, yes — the length × width × depth capacity still holds. For sloped batters or a natural basin the box formula over-estimates volume, so use an average-end-area or survey-based method instead.
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