Water Demand Calculator

Household water demand = (people × indoor use per person) + pets + garden + pool − rainwater offset. Indoor use per person is split into five end uses: drinking and cooking 10 L, showering 52 L, toilet 30 L, faucets 33 L and laundry 25 L, for a planning baseline of 150 L per person per day (WHO guidance). Garden volume = area in m² × watering depth in mm per week ÷ 7 (because 1 mm of water over 1 m² equals 1 liter). Pool loss = surface area × evaporation in mm per day. The calculator above does all of this for you and also estimates the monthly cost from your local water tariff. This water demand calculation formula gives an average-day planning figure, not an engineering design — compare it with your actual bill to calibrate it.

It depends on the country. A common international planning figure is about 150 liters (40 gallons) per person per day for indoor use, based on WHO domestic water guidance. Actual water consumption per person varies: the US averages roughly 300 liters per person per day including outdoor use (USGS), the UK around 140–150 liters (Ofwat), and Australia 100–230 liters depending on the state. Developing-country basic needs can be as low as 50 liters per person per day (WHO minimum). Use the calculator with your own household details for a personalized estimate.

At the 150 L per person planning baseline, a 2-person household uses about 300 liters (79 gallons) per day indoors — roughly 9 cubic meters per month — before any garden or pool use. Add about 1–2 liters per day per pet, and for a 100 m² lawn watered at 25 mm per week, about 357 liters per day extra. A real-world 2-person household with a garden typically lands between 400 and 800 liters per day depending on climate and habits.

Per capita water demand is the average volume of water one person needs per day, usually expressed in liters per capita per day (lpcd) or gallons per capita per day (gpcd). Utilities use it to size water supply systems: for example, 150 lpcd × 10,000 people = 1.5 million liters per day of average demand. It is different from personal drinking-water intake — it covers everything: bathing, toilets, laundry, cooking, cleaning, pets and outdoor use.

The IAPMO Water Demand Calculator (WDC) is a free tool from the International Association of Plumbing and Mechanical Officials that sizes water supply pipes using the fixture-unit method from the Uniform Plumbing Code. It answers a different question than this page: pipe diameter for plumbers and engineers, not how many liters your household consumes per day. If you need pipe sizing for a new build or remodel, use the IAPMO WDC; if you want your daily, monthly and cost estimate, use this calculator.

Crop water demand = reference evapotranspiration (ET₀) × crop coefficient (Kc), from FAO Irrigation and Drainage Paper 56. ET₀ depends on climate: about 6.5 mm/day in hot dry weather, 5.0 in warm, 3.5 in moderate and 2.0 in cool conditions. Kc depends on the crop — rice 1.10, maize 1.15, wheat 1.00, sugarcane 1.20, fruit trees 0.90. One millimeter over one hectare equals 10,000 liters, and you divide by the irrigation method's efficiency (drip 90%, sprinkler 75%, flood 55%) to get the gross water you must actually supply.

A family of 4 uses about 600 liters (159 gallons) per day indoors at the 150 L per person baseline. A typical worked example: 4 people (600 L) + 2 medium dogs (2.2 L) + a 100 m² garden at 25 mm/week (357 L) = about 959 liters per day, or 28.8 m³ per month. At a tariff of $1.50 per m³ that is roughly $43 per month. Enter your own numbers above — real households range from 400 to over 2,000 liters per day once gardens and pools are included.

Monthly cost = total cubic meters used × your local tariff per m³. For example, 28.8 m³ at $1.50/m³ = $43.20 per month. Tariffs vary enormously: many US utilities charge $1–$3 per m³, UK water bills average the equivalent of $2–$4 per m³, and Indonesian PDAM tariffs are often Rp 3,000–Rp 10,000 per m³. Enter your tariff in the calculator (USD, EUR, GBP, IDR, AUD and more are supported) to see your daily, monthly and yearly cost.

The biggest indoor savings come from fixtures: an efficient toilet saves about 30% (30 L → 21 L per person per day), a low-flow showerhead about 25% (52 L → 39 L) and efficient faucets about 20% (33 L → 26 L) — together roughly 114 liters per day for a family of 4. Outdoors, drip irrigation, watering early morning, mulching and rainwater harvesting cut garden demand sharply; rainwater can offset most outdoor use. The calculator's savings panel quantifies all of this in liters and money for your household.

Veterinary guidance is about 50–60 ml of water per kilogram of body weight per day. That works out to roughly 0.5 L for a small dog (9 kg), 1.1 L for a medium dog (20 kg) and 2.2 L for a large dog (40 kg), plus a little extra for washing bowls. Cats need about 0.3 L per day. Pets are a small slice of household demand — two medium dogs add just 2.2 L per day — but the calculator includes them for completeness.

The standard horticultural rule is about 25 mm (1 inch) of water per week, including rainfall. For a 100 m² lawn that is 2,500 liters per week, or about 357 liters per day averaged out — often the single largest item in a household's water demand. Sandy soils and hot climates need more, clay soils and cool climates less. Watering deeply 2–3 times a week is better than daily sprinkling, and the calculator shows the per-watering-event volume for your chosen frequency.

A typical outdoor pool loses about 5–8 mm of water per day to evaporation in warm weather (the calculator defaults to 6 mm), more in hot dry windy climates and less when covered. For a 32 m² pool (8 m × 4 m) at 6 mm/day that is 192 liters per day — over 5.7 m³ per month just from evaporation. A pool cover cuts evaporation by 70–95%, which is the single most effective pool water saver.

Water demand is how much water a household, building or farm needs to function — the planning figure utilities and this calculator estimate. Water consumption is how much is actually used, measured by your meter. Demand is a forecast (liters per day); consumption is a measurement (your bill). They differ because of leaks, behavior, weather and efficiency — which is exactly why comparing this calculator's estimate with your actual bill is useful: a big gap often means a leak or an inefficient fixture.

Maximum daily demand is the highest single-day demand a water system must handle, typically 1.5 to 2.0 times the average daily demand (the 'peaking factor'). Utilities size pipes and treatment plants for the maximum day, not the average. This calculator estimates your average daily demand; multiply by about 1.8 for a rough planning peak if you are sizing a tank, pump or rainwater system. It is an educational estimate, not an engineering design — consult a professional for real system sizing.

It is an educational planning estimate, not a measurement. The factors are published planning figures (WHO 150 L/person/day, FAO-56 crop coefficients, the 25 mm/week lawn rule), and the worked examples are hand-verified. Real use varies with climate, habits, fixture age and soil. Treat the result as a solid starting point for budgeting, comparing tariffs or sizing a rainwater tank — and compare it with your actual water bill to calibrate it to your household.

Water demand vs water intake comes down to scope: a water demand calculator estimates total household or farm water use in liters per day — bathing, toilets, laundry, garden, pool, irrigation — for budgeting and planning. A water intake calculator estimates how much water one person should drink per day for health. Different questions, different tools: use this page for your household's total demand and cost, and our water intake calculator for personal daily drinking targets.