Skip to content
gigabiome
Try Plant Check

Free calculator

Cooling tower water balance calculator

A cooling tower rejects heat by evaporating water. Minerals stay behind, so the tower bleeds some water off (blowdown) and replaces it with fresh makeup. This calculator gives all three flows from the plant's size, the tower's range and its cycles of concentration.

Formula
Makeup = E × C ÷ (C − 1), E = 0.00085 × Q × R
Last reviewed
2026-09-29

Inputs

Example values are pre-filled so you can see the result. Replace them with your plant's.

AHRI rating condition: 3.0

About 10 °F at AHRI rating conditions

0.00085 for average conditions; lower in humid climates, higher in dry ones

Modern drift eliminators: very small

Results

Example values
Makeup
19.13 gpm
Makeup per year
2,295,000 gal
basinMake-up19.13 gpmEvaporation12.75 gpmDrift0.00 gpmBlowdown6.38 gpm

Arrow width is proportional to flow

Circulation
1,500 gpm
Evaporation
12.75 gpm
Blowdown
6.38 gpm
Blowdown per year
765,000 gal
Show the math
  1. Q = tons × gpm per ton= 500 × 3 = 1,500 gpm
  2. E = k × Q × R= 0.00085 × 1,500 gpm × 10 °F = 12.75 gpm
  3. M = E × C ÷ (C − 1)= 12.75 × 3 ÷ (3 − 1) = 19.125 gpm
  4. Blowdown + drift = E ÷ (C − 1)= 12.75 ÷ (3 − 1) = 6.375 gpm
  5. Makeup per year = M × 60 min × hours= 19.125 × 60 × 2,000 = 2,295,000 gal
  6. Blowdown per year = blowdown × 60 min × hours= 6.375 × 60 × 2,000 = 765,000 gal

Annual figures scale design-load rates by equivalent full-load hours.

How it's calculated

Evaporation. E = 0.00085 × Q × R, with E and circulating flow Q in gpm and range R in °F. About 1% of circulation evaporates per 10 °F of range; the 0.85 factor allows for the share of heat that leaves as sensible rather than latent heat. Humid climates run lower and dry ones higher, so the coefficient is an input.

Makeup and blowdown. Cycles of concentration C is the ratio of makeup to blowdown. At steady state, makeup M = E × C / (C − 1) and blowdown plus drift = E / (C − 1).

Flow. Water-cooled chillers are rated with 85 °F entering condenser water at 3.0 gpm per ton (AHRI 550/590), which gives a range of about 10 °F. Use your measured flow if you have it.

Questions

How much water does a cooling tower evaporate?

Roughly 1% of the circulating flow for every 10 °F of range, or 0.00085 × flow (gpm) × range (°F) for average conditions. A 500-ton plant at 3 gpm per ton and a 10 °F range evaporates about 12.75 gpm at full load.

What is the makeup water formula for a cooling tower?

Makeup = evaporation × C / (C − 1), where C is the cycles of concentration. Blowdown plus drift = evaporation / (C − 1).

Why does blowdown matter?

Blowdown is what keeps dissolved minerals from concentrating to the point of scale. It is also water you pay for twice, once as makeup and again as sewer charges in many cities.