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Sump and overhead tank: how to check the litres on a quote

A sump quoted at 6,000 litres can hold 4,800. Work out what a built sump or a plastic tank really stores from its inside size and overflow level, and read a plastic tank's label the way IS 12701 defines it.

By Bulan Sarkar · Updated

Quick Answer

Litres = inside length × inside width × water depth, in metres, × 1,000. The water depth is measured from the floor up to the inlet or overflow level, not to the roof slab and not from the outside of the walls. That gap is why a quote and a sump often differ by a fifth or more. For plastic tanks, IS 12701:1996 defines the label figure as net capacity: the water between the outlet level and the inlet or overflow level. Both levels are marked on the tank with a painted 25 mm band.

Source: IS 12701:1996 Rotational Moulded Polyethylene Water Storage Tanks, Specification, checked Oct 2026.

1 m³= 1,000 litres
1 cft= 28.32 litres
25 mmband painted at the overflow and outlet levels (IS 12701)
5%minimum extra gross over net capacity (IS 12701)

Three different numbers for one sump

A built sump has an outside size, an inside size and a water level, and each gives a different number of litres. Only the last one is water you can use.

  • The outside size includes the walls and floor. Nobody stores water in concrete, so this number is always too big.
  • The inside size to the slab is the full hollow. The top part never fills: water stops at the inlet or the overflow pipe.
  • The inside size to the overflow is what the sump holds. If the pump's foot valve sits above the floor, the water below it stays put too.

When a quote says "6,000 litre sump", ask which of these it means and ask for the inside length, width and the depth to the overflow. Then do the sum yourself.

One sump, four answers to "how many litres?"Outside size 10,440 litres. Inside to the slab 6,000 litres. Inside to the overflow 4,800 litres. Above the foot valve 4,400 litres.Outside size (walls included)10,440 LInside, floor to slab (the quote)6,000 LInside, floor to overflow4,800 L storedAbove the pump's foot valve4,400 L the pump can lift
One sump, four answers to "how many litres?". The sump in the first worked example: outside 2.9 × 2.0 × 1.8 m, inside 2.5 × 1.6 m, 1.5 m floor to slab, overflow 0.3 m below the slab, foot valve 0.1 m above the floor.

What the litres on a plastic tank mean

Most overhead tanks are rotational moulded polyethylene, made to IS 12701:1996. The standard defines the terms you'll see:

  • Net capacity: the water between the lowest level of the inlet and the lowest specified level, which is the outlet. This is the figure in the name: a "1,000 litre" tank means at least 1,000 litres net.
  • Gross capacity: the whole enclosed volume, including space that can't be filled. It must be at least 5% more than the minimum net capacity.
  • Effective height: from the base to the overflow connection, the point that limits how much water the tank stores.

Clause 10 asks for the maker's name, the net capacity in litres, the batch number and the year of manufacture on every tank. The maker's name and the net capacity are also moulded into the outside wall. A painted 25 mm band marks the inlet/overflow level and the outlet level.

IS 12701 also says these tanks are not meant for underground use. A plastic tank sold as a buried sump is outside this standard, so ask the seller which standard it is made to.

Checking a plastic tank at the shop1 read the net capacity and year moulded into the wall; 2 find the two painted bands, overflow and outlet; 3 measure the outside diameter and the height between the bands; 4 work out litres with diameter less 1 cm; 5 compare with the net capacity on the label.1Read the moulded net capacity and the yearClause 10: name, net litres, batch, year2Find the two painted 25 mm bandsInlet/overflow level and outlet level3Measure the outside diameter and the heightbetween the bandsUse the middle of each band4Litres = 0.7854 × D² × H × 1,000D = outside diameter less about 1 cm for the walls5Compare with the labelRough check: curved tops and ribs shift it a little
Checking a plastic tank at the shop. From IS 12701:1996 clauses 3, 5 and 10. A tape and two minutes is enough.

The sum, in metres or in feet

Capacity formulas and unit conversions
Shape or unitLitres
Rectangular sump or loft tank, sizes in metresL × W × water depth × 1,000
Round tank, sizes in metres0.7854 × D² × water depth × 1,000
Sizes in feetL × W × depth (cft) × 28.317
1 cubic metre1,000
1 cubic foot28.317
1 imperial gallon (older Indian quotes)4.546
1 US gallon3.785

Measure the inside with a tape held down through the manhole, or take the sizes off the drawing and check them after shuttering. Measure the water depth from the floor to the bottom of the inlet or overflow pipe.

To decide how many litres you need in the first place, use the water tank calculator. It multiplies people by litres per person per day (135 to start with; IS 1172 sets 150 to 200 for a big city with flush toilets) and by the days of storage you want.

The sum in feet, step by stepInside size 8 by 5 feet. Water depth 4 feet to the overflow. 8 times 5 times 4 is 160 cubic feet. 160 times 28.317 litres is 4,530.7 litres. Call it 4,500 litres when you compare quotes.1Inside: 8 × 5 ftTaped wall to wall through the manhole2Water depth: 4 ftFloor to the bottom of the overflow pipe38 × 5 × 4 = 160 cft4160 × 28.317 = 4,530.7 litres1 cft is 28.317 L5Call it 4,500 LThe figure to compare quotes with
The sum in feet, step by step. The second worked example: a sump 8 × 5 ft inside, with 4 ft of water up to the overflow.

Worked examples

1. A sump quoted at 6,000 litres

  • Inside: 2.5 × 1.6 m; floor to roof slab 1.5 m
  • Floor to slab: 2.5 × 1.6 × 1.5 = 6.0 m³ = 6,000 L (the quote's figure)
  • Inlet and overflow sit 0.3 m below the slab, so water depth is 1.2 m
  • Stored: 2.5 × 1.6 × 1.2 = 4.8 m³ = 4,800 L
  • Foot valve 0.1 m above the floor: 2.5 × 1.6 × 1.1 = 4.4 m³ = 4,400 L the pump can lift
  • Outside size, 2.9 × 2.0 × 1.8 m = 10.44 m³, was never a water figure

Result: The sump stores 4,800 litres, a fifth less than quoted. To store 6,000 litres to the overflow, the inside needs 1.5 m of water depth at 2.5 × 1.6 m, or a bigger plan area.

2. Sizes in feet

  • Inside 8 × 5 ft, 4 ft of water up to the overflow
  • 8 × 5 × 4 = 160 cft
  • 160 × 28.317 = 4,530.7 L

Result: About 4,530 litres. Call it 4,500 when you compare quotes.

3. Checking a 1,000 litre plastic tank

  • Outside diameter 1.12 m; IS 12701 Table 1 wall for 1,000 L is at least 4.5 mm, so use 1.11 m inside
  • Painted bands: overflow 1.10 m above the base, outlet 0.05 m above it, so 1.05 m of water
  • 0.7854 × 1.11 × 1.11 × 1.05 = 1.016 m³
  • = 1,016 L between the bands

Result: About 1,016 litres, so the 1,000 litre net capacity checks out. For a family of four at 135 L a head, that is 1,016 ÷ 540 = 1.9 days of water.

Tips and tricks

  • Ask for the inside size and the overflow depth in writing on the quote. A litre figure on its own can't be checked.
  • Measure from the bottom of the overflow pipe, not its centre. On a 100 mm pipe that's 50 mm of depth, about 200 litres on a 2.5 × 1.6 m sump.
  • On old drawings, "gallons" usually means imperial gallons: multiply by 4.546, not 3.785.
  • Plan the sump to hold more than the overhead tank. The pump refills the tank from the sump, so a sump smaller than the tank runs dry before the tank is full.
  • Check the year moulded on a plastic tank so you know how old the stock is before you pay.

What we would do

We'd size the litres first with the water tank calculator, then write the number into the quote as stored litres to the overflow, with the inside length, width and overflow depth beside it. For a built sump, we'd check those sizes with a tape once the shuttering is up, before the concrete goes in, because that is the last cheap moment to change them. For a plastic tank, we'd read the moulded net capacity and year at the shop and do the band-to-band check before paying.

Frequently asked questions

How do I calculate sump capacity in litres?
Multiply the inside length, inside width and water depth in metres, then multiply by 1,000. Measure the water depth from the floor to the inlet or overflow level. In feet, multiply the cubic feet by 28.317.
What is the size of a 1,000 litre water tank?
IS 12701:1996 Table 1 allows a range for a 1,000 litre vertical tank: an overall diameter of about 1.0 to 1.2 m and an overall height of about 1.05 to 1.35 m, with walls at least 4.5 mm thick. Makers pick sizes inside that range.
What is the difference between net and gross capacity of a water tank?
Net capacity is the water between the outlet level and the inlet or overflow level, the litres you can use. Gross capacity is the whole enclosed volume. IS 12701 asks for the gross to be at least 5% above the minimum net.
Can a plastic water tank be used as an underground sump?
Not under IS 12701:1996, which says its tanks are not meant for underground use. If a seller offers a plastic tank for burying, ask which standard it is made to and follow the maker's installation instructions.
How many cubic feet is 5,000 litres?
5,000 ÷ 28.317 = 176.6 cft. For example, a sump 8 ft × 5.5 ft inside with 4 ft of water holds 176 cft, close to 5,000 litres.

Sources and date checked

This guide covers working out and checking litres. The sump's walls, slab, waterproofing and where it sits relative to the septic tank are for your engineer or plumber. Don't climb into a sump or tank to measure it; measure from the manhole or from the drawings.

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