What the dam percentage actually means
Megalitres, full supply capacity, and why the City and the national department publish slightly different numbers.
A dam’s percentage is simply the water stored divided by its full supply capacity. Capacity is measured in megalitres (Ml). One megalitre is a million litres, roughly the water in an Olympic-size swimming pool. Cape Town’s six supply dams together hold 898 221 Ml when full.
Theewaterskloof is by far the largest at 480 188 Ml, more than half the system. Voëlvlei holds 164 095 Ml, Berg River 130 010 Ml, Wemmershoek 58 644 Ml, Steenbras Lower 33 517 Ml and Steenbras Upper 31 767 Ml. That is why a change at Theewaterskloof moves the system total far more than a change at Steenbras.
Percentages can go above 100% when a dam spills over its full supply level, as Steenbras Upper did in late October.
The City of Cape Town and the national Department of Water and Sanitation (DWS) both publish readings. They use slightly different capacities and reading dates, so the numbers rarely match exactly. The City’s page is updated on weekdays, the DWS page weekly. We show which source you are looking at and never mix the two in one gauge.