What size water tank do I need for a family of 4 in Sri Lanka?

At the NWSDB Standard demand of 135 litres per person per day, a family of 4 uses about 540 L a day. For 2 days of storage that is 1,080 L total. Splitting it as one day overhead and one day sump gives a 1,000 L rooftop tank (the nearest standard size ≥ 540 L) plus a roughly 540 L ground sump.

How many litres of water does one person use per day in Sri Lanka?

It depends on the home. This tool offers three sourced presets: Basic 90, Standard 135 (inside the NWSDB house-connection design band) and High 180 litres per person per day. The ADB design analysis cites about 112 lpcd for a typical urban connection. You can enter any figure from 30 to 400 lpcd under Custom.

What is the difference between an overhead tank and a sump?

A sump is a ground-level (often underground) reservoir that fills directly from the mains, storing water so you keep a supply through cuts and low pressure. A pump lifts water from the sump to the overhead (rooftop) tank, which then feeds taps by gravity. Most Sri Lankan homes need both because mains supply is intermittent.

How do I calculate the capacity of a water tank in litres?

For a rectangular tank, multiply length × width × height in metres to get cubic metres, then multiply by 1,000 for litres. For a cylindrical tank, use (π ÷ 4) × diameter² × height in metres, then × 1,000. The Capacity from dimensions tab does this for you and also shows cubic metres and imperial gallons.

How many days of water storage should a house have?

General practice in areas with scheduled cuts is 1 to 2 days of demand. Two days is a safe default for most Sri Lankan homes; homes on reliable supply may hold one day, while higher-elevation areas with long low-pressure spells sometimes plan for more. The storage horizon is adjustable in the tool from 0.5 to 4 days.

Is a single 1,000 L rooftop tank enough for my house?

For a small family holding one day in the overhead tank, yes — 1,000 L covers about 7 people at Standard demand. But a rooftop tank alone gives no buffer once the mains stop and the tank empties. Pairing it with a ground sump that holds a second day of demand is what keeps taps running through a supply cut.

Does this calculator size the pump or water pressure?

No. This version sizes storage volume only — the overhead tank, the sump and their combined days of supply. Pump head, horsepower and booster selection are a separate engineering problem and are deliberately out of scope. Size the storage here first, then choose a pump to match the lift and flow you need.

Where do the demand figures and tank sizes come from?

Per-capita demand presets follow the National Water Supply & Drainage Board design band and the ADB economic analysis (≈112 lpcd urban). The standard tank sizes — 300, 500, 1,000, 1,500, 2,000, 3,000 and 5,000 L — are the common polyethylene sizes sold locally by S-lon, ARPICO and RRR, used only to snap the recommendation to a size you can actually buy.

Sri Lanka · Utility

Water Tank Size Calculator for Sri Lanka — tank & sump sizing

Find the overhead tank and ground sump your home actually needs from the number of people and their daily water use, and snap the result to a real poly-tank size you can buy. A second mode converts any tank's dimensions to litres. Free, no signup, sources cited.

By Induwara Ashinsana— Executive Director, Ryzera TechnologiesUpdated Jul 17, 2026

Size your water tank & sump

NWSDB / ADB demand basis

Occupants (people)

ppl

Everyone living in the house (1–50).

Daily use per person

Litres per person per day (lpcd). Standard 135 is the NWSDB design band.

Total storage to hold

days

Buffer for supply cuts. Practice is 1–2 days (0.5–4).

Held in the overhead tank

days

Rest goes to the ground sump. Range 0.25–2, ≤ total.

Daily demand

540 L

Overhead tank

1,000 L

Needs 540 L

Ground sump

540 L

≈ 0.80 × 0.67 × 1.00 m

Storage provided

2.9 days

1,540 L

How the storage splits

Component	Basis	Litres
Daily demand	4 × 135 lpcd	540 L
Overhead tank (required)	1 day(s) of demand	540 L
Overhead tank (recommended)	1 × 1000 L standard tank	1,000 L
Ground sump	1.00 day(s) of demand	540 L
Total design storage	2 day(s)	1,080 L

The recommended overhead size is rounded up to the nearest standard poly-tank (300 / 500 / 1000 / 1500 / 2000 / 3000 / 5000 L), so you are never left short. Because of rounding, the tank you buy usually holds a little more than the raw requirement.

Demand presets follow NWSDB house-connection design and ADB's ≈112 lpcd urban figure; 1 imp gallon = 4.54609 L, 1 m³ = 1,000 L. Sources cited in full below.

How it works

The tool has two independent calculators. The first sizes storage from demand; the second converts dimensionsto capacity. All lengths are handled internally in metres (1 ft = 0.3048 m), with 1 m³ = 1,000 L and 1 imperial gallon = 4.54609 L.

Sizing a tank from demand

Daily demand D = occupants × lpcd. The litres-per- person-per-day (lpcd) presets — Basic 90, Standard 135, High 180 — follow the National Water Supply & Drainage Board design band for house connections. The ADB economic analysis cites about 112 lpcd for a typical urban connection, which you can enter under Custom.
Total storage S = D × storage days. Because mains supply in Sri Lanka is frequently interrupted by scheduled cuts and low pressure, households store 1–2 days of demand as a buffer. The default is 2 days; you can set anywhere from 0.5 to 4.
Overhead volume O = D × overhead days (capped at S), and ground sump G = S − O. The overhead tank feeds taps by gravity; the sump holds the reserve the pump lifts up as the tank drains.
The recommended overhead size is the smallest standard poly-tank whose capacity is at least O, chosen from 300 / 500 / 1,000 / 1,500 / 2,000 / 3,000 / 5,000 L. Rounding is always upso a household is never left short; above 5,000 L the tool reports how many 5,000 L tanks are needed.
The sump is reported as litres, cubic metres and a suggested rectangular build. Depth is fixed at a practical 1.0 m and the footprint is solved from G with a 1.2:1 length-to-width ratio, so a 0.675 m³ sump becomes about 0.90 m × 0.75 m × 1.0 m.

Capacity from dimensions

A rectangular tank holds V = L × W × H cubic metres; a cylindrical tank holds V = (π ÷ 4) × d² × h. Both are multiplied by 1,000 for litres and divided by 4.54609 for imperial gallons. The cylinder result is cross-checked against the radius form V = π · r² · h, which is algebraically identical, so the two agree to the litre. Enter a target capacity and the tool solves one practical set of dimensions in reverse.

The demand figure and storage horizon are transparent, user-adjustable design assumptions — not a statutory rate — so the tool never publishes a hidden number. The sources were last checked on 2026-07-17.

Worked examples

Family of 5 · Standard urban · 2-day storage

Daily demand: 5 × 135 lpcd = 675 L/day
Total storage: 675 × 2 days = 1,350 L
Overhead (1 day): 675 L → nearest standard ≥ 675 = 1,000 L tank
Sump (1 day): 1,350 − 675 = 675 L ≈ 0.68 m³
Sump build: ≈ 0.90 m × 0.75 m × 1.0 m (depth 1.0 m)

Household of 3 · ADB typical urban 112 lpcd

Daily demand: 3 × 112 lpcd = 336 L/day
Total storage: 336 × 2 days = 672 L
Overhead (1 day): 336 L → nearest standard ≥ 336 = 500 L tank
Sump (1 day): 672 − 336 = 336 L ≈ 0.34 m³
Result: one 500 L rooftop tank plus a small ground sump

Dimensions → capacity · cylindrical rooftop tank

Diameter 1.0 m, height 1.2 m
V = (π ÷ 4) × 1.0² × 1.2 = 0.9425 m³
Capacity: 0.9425 × 1,000 = 942.5 L
In gallons: 942.5 ÷ 4.54609 ≈ 207.3 imperial gallons
Radius-form check: π × 0.5² × 1.2 × 1,000 = 942.5 L (matches)

Frequently asked questions

Sources & references

Per-capita demand presets and the standard poly-tank sizes were last checked against these sources on 2026-07-17. The lpcd figure and storage horizon are adjustable design assumptions, shown openly, not a statutory rate.

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