Both calculators put a number on household water, but on different sides of the ledger. The evaporation rate calculator estimates how fast an open water surface — a pool, pond, tank, or reservoir — loses water to the air from temperature, humidity, and wind. The water demand calculator totals how much water a household uses from people and fixture events, with an optional annual cost at your water-bill rate. One measures a weather-driven loss; the other measures a consumption-driven need.
What each calculator does
The evaporation rate calculator estimates open-water loss from air temperature, water temperature, relative humidity, wind speed, and surface area. It computes saturation vapor pressure at the water temperature and actual vapor pressure from air temperature and relative humidity; the difference between them is the driving force, and a wind term (with a small minimum wind speed) multiplies it. The result is reported in liters per hour, gallons per day, and an indicated depth loss, and it is clamped to zero when the formula yields a negative or tiny value — which can happen in saturated air or when the water is cooler than the air. The article is explicit that this is a simplified vapor-pressure and wind relationship for an open water surface, not the FAO Penman-Monteith reference evapotranspiration method used for crop and irrigation planning.
The water demand calculator totals household use. You enter people, events per person-day, gallons per event, any other measured gallons per day, and an optional cost per 1,000 gallons. Daily gallons equal people × events per person-day × gallons per event plus other measured gallons; annual gallons multiply the daily total by 365; and cost applies the entered rate. The article’s worked example — two people, five events per day at 1.6 gallons each — comes to 16.0 gallons per day and 5,840 gallons per year. The article makes no savings or efficiency claim: it totals user-entered values, and the inputs should come from bills, measurements, or fixture labels.
Side-by-side
| Evaporation rate calculator | Water demand calculator | |
|---|---|---|
| Question | How much water does an open surface lose to the air? | How much water does a household use? |
| Key inputs | Air temp, water temp, relative humidity, wind, surface area | People, events/person-day, gallons/event, other measured gal/day, $/1,000 gal rate |
| Method | Saturation and actual vapor pressure, vapor pressure deficit, wind term | People × events × gallons/event + other measured use, annualized × 365 |
| Outputs | Liters/hour, gallons/day, depth loss (clamped at zero) | Daily and annual gallons, annual cost at the entered rate |
| Driving force | Weather: humidity, temperature, wind | Activity: fixture events and measured use |
| Cost estimate | None | Optional, at the entered rate per 1,000 gallons |
When to use which
Use the evaporation rate calculator when you are checking a specific open-water surface: how much a pool, pond, tank, or reservoir is likely to lose under a given set of weather conditions, and whether measured losses look reasonable. It is a weather-based estimate, not a leak test — if measured water loss greatly exceeds the estimate, leaks or plumbing losses may need investigation, and pool evaporation also depends on covers, heating schedules, splash-out, wind shelter, solar gain, and night cooling.
Use the water demand calculator when you are quantifying consumption: daily and annual household use from real fixture rates and measured use, or what your water habits cost at your bill rate. Enter values from fixture labels, meter reads, or bills, and treat the scenario ranges as sensitivity ranges rather than confidence intervals.
The two tools meet in outdoor water planning: the demand page lists the rainwater harvesting calculator among its related household resource tools, and the evaporation estimate supplies the loss side of an open surface such as a pool. Use each for the side of the water ledger it measures, and combine them when you need both a loss figure and a use figure for the same property.
Limits and disclaimer
Both pages are educational scenario tools, not audits or guarantees. The evaporation estimate is a simplified vapor-pressure and wind model — not the FAO reference evapotranspiration method — and does not model net radiation, heat storage, salinity, covers, shading, or splash, and the form clamps negative rates to zero. The water demand estimate makes no climate, efficiency, WaterSense, or savings claim and should be built from bills, measurements, or fixture labels, not assumptions. Neither result establishes code compliance, certification, forecast accuracy, or model validity; for irrigation scheduling and agricultural water management, standardized reference evapotranspiration methods are more appropriate than the open-water estimate.