Water Tank Capacity Calculator in Litres
Water tank capacity (litres) = internal length × breadth × (height − freeboard) × 1,000, all in metres. A 2 × 1.5 m tank, 1.5 m deep with 0.2 m freeboard, holds 2 × 1.5 × 1.3 × 1,000 = 3,900 litres.
In feet, litres = cft × 28.3168. The calculator below also covers sumps, upright round tanks, household storage and IS 2470 septic tank sizes for up to 20 users (WC only).
Calculate tank capacity or household storage
For feet and inches, use total inches (5 ft 6 in = 66 in) or decimal feet (5.5 ft). Changing units converts the current dimensions.
- Water capacity below freeboard
- 3,900 L
- Water volume
- 3.9 m³ / 137.73 cft
- Water depth
- 1.3 m
- Gross internal capacity
- 4,500 L
- Space reserved as freeboard
- 600 L
Measure inside the tank. Capacity is to the chosen water level; outlet level, pump cut-off and dead storage may reduce usable supply. Cylindrical mode is for an upright cylinder.
Capacity from dimensions: rectangular, cylindrical and sump
Sump capacity uses the same formula as any water tank: internal plan area × water depth. Freeboard is the air gap above the top water level, so take it off the internal height first.
- Rectangular: V = L × B × (H − F). With metres, litres = V × 1,000.
- Upright cylinder: V = π × D² ÷ 4 × (H − F), with D the internal diameter.
- In feet: litres ≈ L × B × (H − F) × 28.3168. The calculator uses 1 ft = 0.3048 m exactly.
- Feet and inches: 5 ft 6 in is 5.5 ft or 66 in, not 5.6 ft.
A round tank 2 m in internal diameter and 1.5 m high, with 0.2 m freeboard, holds π × 1² × 1.3 × 1,000 ≈ 4,084.07 L. Domed tops and tapered walls are not modelled; for a plastic tank, go by the maker’s rated capacity.
An 8 × 5 × 5 ft sump with 6 in freeboard has 4.5 ft of water: 8 × 5 × 4.5 = 180 cft ≈ 5,097.03 L. Water below the pump suction or outlet cannot be drawn, so usable capacity is a little less.
Required storage = persons × demand × days
Five people at 135 L/person/day with two days of storage need 5 × 135 × 2 = 1,350 L. All three figures are inputs you can change.
135 lpcd is not a code figure for every house. IS 1172:1993, clause 4.1 gives 150–200 lpcd for towns above 1 lakh people with full flushing; its note allows 135 lpcd for LIG and EWS houses, depending on local conditions. Use the demand your local body or plumbing designer accepts.
Two days of storage and a 40% overhead share are our starting assumptions, not code ratios. With them, 1,350 × 0.40 = 540 L overhead and 810 L in the sump. Set the days to the longest gap in your supply, and the split to your pumping arrangement.
At an assumed water depth of 1.2 m, the combined internal footprint is 1.35 ÷ 1.2 = 1.125 m², or a single square tank of √1.125 ≈ 1.061 m sides. Add freeboard above that depth and wall thickness outside the plan. For a sump plus overhead tank, size each one separately.
Keep fire storage separate. A structural engineer should check the roof or tower before you fix an overhead tank size.
Septic tank size by number of users (IS 2470 table)
A septic tank for 5 users, WC waste only and cleaned once a year, is 1.5 m × 0.75 m with 1.0 m liquid depth = 1,125 litres, plus 0.3 m freeboard.
These internal sizes are from IS 2470 (Part 1):1985, Appendix A, Table 5. The table assumes only WC discharge goes to the tank, and its notes ask for exact calculations when choosing a size.
| Users | L × B (m) | Liquid depth: 1 / 2 years (m) | Liquid capacity: 1 / 2 years (L) |
|---|---|---|---|
| 5 | 1.5 × 0.75 | 1 / 1.05 | 1,125 / 1,181.25 |
| 10 | 2 × 0.9 | 1 / 1.4 | 1,800 / 2,520 |
| 15 | 2 × 0.9 | 1.3 / 2 | 2,340 / 3,600 |
| 20 | 2.3 × 1.1 | 1.3 / 1.8 | 3,289 / 4,554 |
Internal height = liquid depth + 0.3 m. Between rows, say 7 users, the calculator takes the next row up; that is our lookup choice, not an IS rule. It stops at 20 users.
Bath, kitchen and laundry water, or a different cleaning interval, need an engineer to work out flow, settling and sludge storage, and to check the soil and the soak pit or dispersion trench.
RCC sump quantities and septic tank construction cost
Tank capacity is the water space, not the RCC quantity. Measure the base, walls and cover slab separately from the structural drawings.
Use the concrete calculator for measured RCC volumes and the BBS calculator for steel from bar sizes, spacing and cutting lengths. Wall thickness and reinforcement need a structural design; capacity alone cannot give a reliable steel weight.
Septic tank construction cost also covers excavation, access covers, pipes, waterproofing and the soak pit or other effluent disposal. Price a measured BOQ at local rates, or send the drawings for a BOQ from drawings. The residential plumbing BOQ guide lists the water supply and drainage items.
Water tank and septic tank FAQs
How do I calculate water tank capacity in litres?
Multiply internal length × breadth × water depth in metres, then × 1,000. Water depth is internal height minus freeboard. A 2 × 1.5 m tank, 1.5 m deep with 0.2 m freeboard, holds 2 × 1.5 × 1.3 × 1,000 = 3,900 litres.
How many litres are in one cubic foot?
One cubic foot is 28.316846592 litres, usually rounded to 28.3168. Multiply the water volume in cft by this figure, using internal dimensions, not the tank’s outside size.
What size water tank does a family of five need?
About 1,350 litres, assuming 135 litres per person per day and two days of storage. Change both to match your water supply. Freeboard and water below the outlet are extra.
Is 135 litres per person per day mandatory for every Indian house?
No. IS 1172:1993 clause 4.1 sets residential demand by town size and flushing: 150–200 L/person/day for towns above 1 lakh people with full flushing. Its note allows a reduction to 135 for LIG and EWS houses, depending on local conditions. Confirm the figure with your local authority or plumbing designer.
How should I split water storage between sump and overhead tank?
Set the overhead percentage yourself. The 40% starting value is our assumption, not a code requirement. The right split depends on supply timing, pumping, the roof structure and backup needs. Keep fire storage separate.
Can I use the septic tank size calculator for all household wastewater?
No. IS 2470 Part 1 Table 5 assumes WC discharge only, for up to 20 users. Bath, kitchen and laundry water, more users or a different cleaning interval need a separate design. An engineer should check the tank and the soak pit or other effluent disposal.
Does tank capacity tell me the RCC or septic tank construction cost?
No. Water volume is not concrete volume. Cost needs measured excavation, base and walls, reinforcement, covers, waterproofing, pipes and effluent disposal, priced at local rates.