Work backwards from demand, then check the catchment can fill it — four steps. Step one, demand: add up daily water needs — a dairy cow drinks 60–100 litres, a beef animal 40–60, goats and sheep 5–10, poultry less than a litre each, a household 100–200 — and multiply by the length of your dry gap in days. A 20-cattle herd plus a homestead across a 150-day gap needs roughly 250–350 m³. Step two, losses: add an allowance of 20–30% for evaporation and seepage — pans in hot lowland counties lose more than highland ones. That takes the example to 300–450 m³; round to a buildable 500 m³. Step three, geometry: a bowl of roughly 30 by 20 metres at 1.5 metres average depth, with 1:2 side slopes and a metre of freeboard, delivers about that storage. Step four, the catchment check: dependable yield equals rainfall × catchment area × runoff coefficient — use 10–15% for rangeland, 15–25% for cultivated or rocky ground. Filling 500 m³ on a 15% coefficient needs the seasonal runoff from about 3,300 m² of catchment — so either a catchment area ten times the pan's or a diversion channel from uphill. If the check fails, enlarge the catchment or the pan; if it passes, build the size demand chose, not the size hope chose.

Working figures for planning, rising with heat and lactation: dairy cattle 60–100 litres per head daily — the top of the band for high-yielders in hot weather; beef cattle and draught animals 40–60; goats and sheep 5–10; pigs 15–25; poultry 0.3–0.5 per bird; camels effectively variable — they drink episodically in large gulps, so trough design matters more than daily arithmetic. Add the homestead — 100–200 litres per household daily — and any irrigation: horticulture under drip runs 400–800 m³ per hectare per season, which dwarfs livestock numbers wherever both appear. Multiply daily totals by the dry gap: the difference between a 90-day and a 180-day gap is the difference between a pan and a small dam. The full dairy-farm application of these tables — including trough flow and herd-management details — sits in our guide to water pan excavation for dairy farming.

The rule of thumb says ten — a catchment area around ten times the pan's surface area — but the honest answer is arithmetic. Dependable yield equals rainfall × catchment area × runoff coefficient, and the coefficient is where honesty lives: 10–15% for permeable rangeland and bush, 15–25% for cultivated, compacted or rocky ground, with clean rock approaches higher. Worked: a pan needing 500 m³ of storage in a 600mm rainfall zone on 15% rangeland must harvest 3,300 m³ of runoff, which needs 3,300 m² of effective catchment — the ten-times rule in action. A pan with a smaller natural catchment needs help: a diversion channel or cutoff drain intercepting uphill runoff and leading it home, sometimes doubling the effective area. And the converse is worth stating: a generous catchment feeding a small pan fills fast and spills often — spillway design then governs, because the pan that fills quickly must also empty safely. The sizing survey measures the catchment before the digger measures the pan; that order is worth more than any machine.

Kenya's open-water evaporation runs roughly 1,800–2,200mm per year — 1.8 to 2.2 metres of water depth — varying with altitude and humidity: the hot lowlands (Turkana, Tana River, lower Eastern) sit at the top of the band, the highlands at the bottom. On a pan holding 1.5 metres average depth, that means a full pan's worth of surface water can evaporate in a good year — which is why depth, not surface area, is the farmer's friend: every extra metre of depth adds a year's storage for the same evaporating skin. Practical allowances for planning: add 20–30% to computed demand for evaporation and seepage combined — the lower end on deep, lined, highland pans; the upper on shallow, unlined, lowland ones. And the operating disciplines that shave the loss: build deep with steep-ish stable batters, keep the freeboard real, control floating vegetation that adds transpiration on top of evaporation, and where a pan serves critical dry-season supply, a floating cover or shade structure on a small pan pays back faster than any liner.

Irrigation dominates sizing arithmetic wherever it appears: horticulture under drip consumes 400–800 m³ per hectare per season, and even conservative supplementary watering of a hectare can out-drink a cattle herd tenfold. The method: total the seasonal irrigation requirement per hectare by crop — tomatoes, onions and vegetables toward the top of the band, drought-tolerant crops lower — multiply by the area actually irrigated in the dry window, add livestock and household demand on top, and add the 20–30% losses allowance. Two worked scales: a half-hectare horticulture block plus 20 cattle and a homestead lands near 700–900 m³ of storage — a pan around 35 by 25 metres at 1.5 metres average depth. Two hectares of drip vegetables plus a 50-cow dairy unit crosses 3,000 m³ — small-dam territory, at which point the design conversation upgrades to zoned embankments, engineered spillways and the full dam engineering in our excavation cost guide and lining comparison. The pattern: irrigation turns pans into dams quickly, and the catchment check becomes decisive long before the concrete does.