One unit of water is one cubic metre, which is 1,000 litres. This is the National Water Supply and Drainage Board’s own definition. Its published tariff states it plainly, in brackets, right where consumers are categorised:
“One Unit shall be defined as one cubic meter. ( 1,000 Liters)” — NWSDB water tariff
So if your bill says 15 units, you used 15,000 litres that month — about 500 litres a day.
One unit is not one litre
A lot of pages online — including an earlier version of this one — state that a unit of water in Sri Lanka means a single litre. That is wrong, and it is wrong by a factor of a thousand.
The mistake matters because it makes a normal bill look impossible. If a unit were a litre, a household billed for 15 units would have used fifteen litres in a month, less than two buckets. Anyone doing that sum concludes their meter is broken or they have been overcharged. The meter is fine: the unit is simply a much larger quantity than the word suggests.
| If your bill says | You used |
|---|---|
| 1 unit | 1,000 litres |
| 5 units | 5,000 litres |
| 10 units | 10,000 litres |
| 15 units | 15,000 litres |
| 20 units | 20,000 litres |
| 30 units | 30,000 litres |
The same convention applies to water billing in most countries. A cubic metre is the standard billing unit, and 1 m³ holds exactly 1,000 litres, because a cubic metre is a box one metre on each side and each litre is one thousandth of that.
What one unit of water costs
Domestic consumers other than Samurdhi recipients and tenement gardens are billed on rising bands. The more you use, the more each additional unit costs — and the monthly service charge rises with your consumption band too.
| Units used in the month | Rate per unit | Monthly service charge |
|---|---|---|
| 0–5 | Rs. 60 | Rs. 300 |
| 6–10 | Rs. 80 | Rs. 300 |
| 11–15 | Rs. 100 | Rs. 300 |
| 16–20 | Rs. 110 | Rs. 400 |
| 21–25 | Rs. 130 | Rs. 500 |
| 26–30 | Rs. 160 | Rs. 600 |
| 31–40 | Rs. 180 | Rs. 1,500 |
| 41–50 | Rs. 210 | Rs. 3,000 |
| 51–75 | Rs. 240 | Rs. 3,500 |
| 76–100 | Rs. 270 | Rs. 4,000 |
| Over 100 | Rs. 300 | Rs. 4,500 |
Source: NWSDB water tariff, the schedule the Board publishes on its current tariff page. Tariffs are set by gazette and can be revised, so check the Board’s page before relying on these figures for anything that matters.
Note how steep the service charge becomes. Crossing from 30 units to 31 units moves the monthly service charge from Rs. 600 to Rs. 1,500 — a Rs. 900 jump for one extra unit of water. If your household sits near a band boundary, a small reduction in use can be worth much more than the price of the water itself.
How to read your water meter
The meter counts the same units your bill charges for.
- Black or white digits — these count whole cubic metres, which are the billable units. This is the number that matters.
- Red digits — these count fractions of a unit, normally litres. They are useful for spotting a leak but are not billed on their own.
- Subtract last month’s reading from the current reading. The difference is the units you consumed.
To check for a leak, turn off every tap and appliance and watch the red digits. If they keep moving, water is escaping somewhere between the meter and the house — and because a leak runs continuously, it can push you into a higher band and a higher service charge at the same time.
Working out a bill
Take a household that used 18 units in a month. The bands are cumulative, not a single flat rate applied to the whole amount:
- First 5 units at Rs. 60 = Rs. 300
- Units 6–10, 5 units at Rs. 80 = Rs. 400
- Units 11–15, 5 units at Rs. 100 = Rs. 500
- Units 16–18, 3 units at Rs. 110 = Rs. 330
- Usage subtotal = Rs. 1,530
- Monthly service charge for the 16–20 band = Rs. 400
- Total before any sewerage charge, taxes or arrears = Rs. 1,930
That household used 18,000 litres. Sewerage charges apply separately where the property is connected, and taxes and any arrears are added by the Board, so your printed bill will not match this arithmetic exactly. Use it to understand the shape of the calculation rather than to predict the final figure.
Where the confusion comes from
“Unit” is doing different work in different places, which is why the question gets asked so often:
- On a water bill, a unit is a cubic metre — 1,000 litres.
- On an electricity bill, a unit is a kilowatt-hour, an entirely unrelated quantity.
- On a bottle of water, the volume is printed in litres or millilitres, and nobody calls it a unit.
- In a water tank specification, capacity is normally given in litres, so a 1,000-litre tank holds exactly one billable unit.
That last comparison is the most useful mental picture. If you have a 1,000-litre tank on your roof, one unit on your bill is one full tank.
Last verified: 3 August 2026 against the NWSDB published tariff and its current tariff page.
