formula.expertYour record

Percent Blowdown

%B=BM×100\%B = \frac{B}{M} \times 100
sig figs

Worked example: 20 L/min bleed against 30 gpm makeup → 17.61%. Press Try an example to run it live, then adjust anything.

Enter your known values, leave one input blank, and solves for the missing one. Tap a variable’s symbol to see what it means, with a typical value. Change any unit and the answer follows.

Where Percent Blowdown fits

Missing an input?
In this toolkit

See also

Cooling Tower Makeup Water Rate: Both use blowdown rate and makeup water rate.Cycles of Concentration (COC = M/B): Both use blowdown rate and makeup water rate.Blowdown Rate from Cycles: Both use blowdown rate.Chemical-Consuming Loss Rate: Both use blowdown rate.Holding Time Index: Both use blowdown rate.
Learning zone

Percent Blowdown explained

MB%B

Percent blowdown is cycles of concentration turned inside out: since COC = M/B, the blowdown share of makeup is simply 100/COC. Five cycles means 20% of every gallon you buy goes down the drain untouched; ten cycles means 10%. Plant managers understand the percentage far faster than they understand cycles, and it is the number that belongs on a water-conservation report.

Use it as a sanity check on a service call. Meter the makeup for an hour, meter the bleed, and if the bleed is running 33% you are at three cycles no matter what the controller display claims: usually because the bleed solenoid is passing, the conductivity probe is fouled with biofilm and reading low, or someone left the manual bypass cracked open. A stuck-open bleed valve is the single most common cause of a mysteriously high water bill on a tower, and this one ratio finds it in ten minutes.

Percent Blowdown formula

%B=BM×100\%B = \frac{B}{M} \times 100
Where
  • %B\%B= Percent blowdown (%)
  • BB= Blowdown rate (L/min)
  • MM= Makeup water rate (L/min)
Where this comes from

Worked from first principles: a definition, a balance or unit arithmetic, so it needs no outside source.

In this toolkit

The cooling tower water balance

6 formulasEvaporation, blowdown, drift and makeup — the four flows that have to balance on every open recirculating tower, and the cycles that link them.

Revised 16 October 2026
Cite this page

formula.expert. (2026, October 16). Percent Blowdown Formula & Calculator. https://formula.expert/solvers/cooling-tower-percent-blowdown

formula.experthttps://formula.expert/solvers/cooling-tower-percent-blowdown