How do you calculate make-up water rate?

Divide total make-up water added by the number of operating days, then multiply by a meter factor. For 4,200 gallons over 30 days with a factor of 1, the rate is 140 gallons per day.

What is a normal make-up water rate for a pool?

It varies with surface area and bather load, but a well-covered commercial pool losing more than about 2 to 4 percent of its volume per day is usually flagged for a leak inspection. Compare your gallons-per-day against the pool's total volume to judge it.

Does make-up water include backwash?

Yes. Total make-up water should capture every source of loss you replace, including backwash discharge, splash-out, partial drains, leaks, and evaporation replacement.

Why is my make-up water rate suddenly higher?

The most common causes are a new plumbing or shell leak, a filter backwashing too frequently, higher bather load with more splash-out, or hot dry weather driving evaporation up.

What is the meter calibration factor for?

Use 1 for a trusted direct meter reading. If your meter reads consistently high or low, enter a factor to correct the bias, for example 0.97 for a meter that over-reads by 3 percent.

Make-up water rate vs evaporation loss, what is the difference?

Evaporation loss is only the water that vaporizes from the surface. Make-up water rate is the total of all losses you replace, so it is always equal to or larger than evaporation alone.

Pool, Spa & Water Treatment Chemistry calculator

Pool Make-up Water Rate Calculator

Make-up water rate tells you how many gallons per day a pool or spa consumes to replace losses from backwash, splash-out, partial drains, leaks, and evaporation. Aquatics operators and facility managers use it to benchmark water efficiency, catch hidden plumbing leaks, and budget for water and sewer costs. A stable rate is a health signal for the vessel; a sudden climb usually means a leak or a fouled filter cycling too often. It is also the baseline for calculating chemical dilution, since every gallon of fresh water dilutes sanitizer and stabilizer.

What this calculator does

Estimate make-up water need from backwash, splash-out, evaporation, leaks, and operating days.
Use it for water purchasing, salt dilution planning, or facility water-use reporting.
It computes the average daily volume of fresh water added to a pool or spa over a reporting period, adjusted for meter bias.

Formula used

Make-up water rate = total make-up volume / operating days x factor

Inputs explained

Total make-up water added: Include backwash, splash-out, partial drains, leaks, and evaporation replacement.
Reporting period length: Use the number of days in the reporting or planning period.
Meter calibration factor: Use 1 for direct measurements or adjust for known meter bias.

How to use the result

Use it at the end of each reporting month, or whenever water bills jump, to establish a per-day consumption baseline and screen for leaks.
An average rate masks day-to-day spikes, so a single large drain-and-refill event can inflate the figure and hide the true steady-state loss.

Current U.S. benchmarks

Global copper trades at $13,552 per tonne (IMF via FRED, Jun 2026), up 37.8% in a year, and U.S. industrial electricity averages 8.66 cents per kWh. Both feed electrified-hardware unit economics.

Common questions

How do you calculate make-up water rate? Divide total make-up water added by the number of operating days, then multiply by a meter factor. For 4,200 gallons over 30 days with a factor of 1, the rate is 140 gallons per day.
What is a normal make-up water rate for a pool? It varies with surface area and bather load, but a well-covered commercial pool losing more than about 2 to 4 percent of its volume per day is usually flagged for a leak inspection. Compare your gallons-per-day against the pool's total volume to judge it.
Does make-up water include backwash? Yes. Total make-up water should capture every source of loss you replace, including backwash discharge, splash-out, partial drains, leaks, and evaporation replacement.
Why is my make-up water rate suddenly higher? The most common causes are a new plumbing or shell leak, a filter backwashing too frequently, higher bather load with more splash-out, or hot dry weather driving evaporation up.
What is the meter calibration factor for? Use 1 for a trusted direct meter reading. If your meter reads consistently high or low, enter a factor to correct the bias, for example 0.97 for a meter that over-reads by 3 percent.

Last reviewed 2026-05-12.