Sri Lanka Water Bill Calculator (NWSDB)

Sri Lanka's water tariff is set by the Minister of Water Supply under Section 84 of the National Water Supply and Drainage Board Law No. 02 of 1974 and gazetted as an Extraordinary Notification. The schedule the calculator uses is from Gazette Extraordinary No. 2343/28 of 02 August 2023, effective from 01 August 2023.

The structure is an increasing block tariff (IBT). One unit equals one cubic metre — 1,000 litres. Your monthly consumption cascades through a series of blocks: the first 5 units at one rate, the next 5 at a higher rate, and so on. NWSDB then adds a monthly service charge whose value is set by the slab your total usage lands in. That fixed component is small at low usage (Rs 300 below 16 units) and rises steeply for heavy users (Rs 4,500 above 100 units a month).

1. Read your meter at the start and end of the month. The difference in cubic metres is your consumption.
2. Pick the right tariff table for your connection — domestic (categories 10/11/13/16/18/19), Samurdhi/tenement (14/17/20/24), schools and religious institutions (12/15/81/85), government hospitals (63), government and non-BOI industries (60/61/62/64/73/75), or commercial including tourist hotels and BOI industries (65/70/71/74/77/79/80/82/98). Your customer category number is printed on the bill near the account header.
3. Cascade the units through the table's blocks. Multiply the units in each block by that block's rate; sum to get the energy charge.
4. Add the slab's monthly service charge — the slab is decided by the total units, not the marginal unit. A customer who uses zero units still pays the lowest slab's service charge (the minimum bill, per Misc. cl. 2 of the gazette).
5. If the premises is inside the NWSDB piped-sewerage area, add the sewerage surcharge. The calculator accepts a user-entered percentage so the result matches the printed bill exactly; v1 does not auto-pick the rate because the sewerage gazette is image-only.

The methodology has an equivalent explicit if/then-else form that short-circuits the block walk. The calculateDomesticByLadder function in lib/data/sri-lanka-water-bill-calculator.ts must always produce the same total to the rupee as the block-cascade path — verified at typecheck time against the eighteen hand-computed reference cases recorded in the file header.

To turn that rate ladder into the question most people actually ask — how much will my bill be?— the table below lists the full domestic water charge (cascaded energy plus the slab's monthly service charge, no sewerage) at the usage levels households hit most often. Each figure is one of the calculator's self-tested reference cases, so it matches the tool above to the rupee. The effective rate column is the total divided by the units, which is why it always sits below the marginal block rate at that level of use.

These are domestic single-meter charges before any sewerage surcharge or VAT. A connection in the Greater Colombo sewerage area adds the sewerage uplift on top; a Samurdhi-registered household pays much less, because the subsidised Tariff Table 01 starts at Rs 5 per unit.

Two design choices in the gazette catch people out. First, the slab that sets the monthly service charge is decided by total consumption, not the marginal unit — so the jump from 30 to 31 units pushes the whole bill into the 31–40 slab and its Rs 1,500 service charge, not just the single extra unit. Second, the cascade is cumulative: a 22-unit household is charged Rs 60 on its first 5 units even though its top units cost Rs 130, because every earlier block is billed at its own lower rate. The effective rate per cubic metre therefore always sits below the marginal block rate until usage is very high.

Shared meters are the main edge case. For a tenement garden or condominium where several households draw from one NWSDB meter, the board divides total consumption by the number of housing units to find the per-household equivalent, reads the slab off that smaller figure, and then scales the energy and service charges back up. A 60-unit meter shared by four flats is billed as four households of 15 units each — keeping each flat in the low Rs 300 service-charge slab — rather than as one 60-unit connection paying the Rs 3,500 slab. Enter the number of households in the “Housing units” field to apply this correctly.

Water is only one of the two metered utilities on a Sri Lankan household budget. Pair this with the Sri Lanka Electricity Bill Calculator to estimate your full monthly utility cost, and if you are budgeting take-home pay against these fixed costs, the Sri Lanka Income Tax Calculator works out your net salary after PAYE. If you are setting up a brand-new supply rather than estimating an ongoing bill, the one-time cost is covered by the Sri Lanka Water Connection Fee Calculator.

• Gazette Extraordinary No. 2343/28 of 02 August 2023 — full tariff schedule (PDF)
• NWSDB — Current Water Tariff page
• NWSDB — Current Sewerage Tariff page
• Public Utilities Commission of Sri Lanka — economic regulator
• National Water Supply and Drainage Board — official site

Calculations follow the Gazette Extraordinary 2343/28 of 02 August 2023, effective 2023-08-01. Last cross-checked on 2026-05-12. The page is re-verified whenever the Minister of Water Supply issues a new tariff gazette under Section 84 of the NWSDB Law.