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Updated Sep 18, 2026

Cooling Tower Wet-Bulb Margin Calculator — Approach, Design Margin & Effectiveness

Compare measured cooling-tower leaving-water temperature with current and design wet-bulb conditions to expose approach, weather margin and thermal effectiveness.

Live calculationWorking shownPrivate in browser

Inputs

Results

Range 6.00 °C
Current approach 3.00 °C
Effectiveness 66.7 %
Design-WB approach at same cold water 1.00 °C
Wet-bulb margin 2.00 °C
More actions

Shared links reopen these inputs. Recent and saved calculations stay in this browser unless you clear site data.

Formula

approach=Tcold−Twb; effectiveness=(Thot−Tcold)/(Thot−Twb).

Your calculation, step by step

These steps use the values currently entered above so the result can be checked independently.

  1. 1.Range = hot water − cold water = 35.00 − 29.00 = 6.00 °C.
  2. 2.Current approach = cold water − current wet bulb = 29.00 − 26.00 = 3.00 °C.
  3. 3.Effectiveness = range ÷ (hot water − wet bulb) = 6.00 ÷ (35.00 − 26.00) × 100 = 66.67%.
  4. 4.At the same cold-water temperature, design-WB approach = 29.00 − 28.00 = 1.00 °C.
  5. 5.Design wet-bulb margin over current = 28.00 − 26.00 = 2.00 °C.

How to use this calculator

  1. Enter measured or design values using the units shown beside each field.
  2. Review the live results and the substituted calculation steps before using the estimate.
  3. Check the assumptions below, then continue to a related calculator when the next engineering decision needs another model.

Worked example

At 35°C hot, 29°C cold and 26°C wet bulb, range is 6°C, approach 3°C and effectiveness about 66.7%.

Assumptions and limits

  • Use representative entering-air wet-bulb measurements.
  • The same-cold-water design-wet-bulb comparison is only physically meaningful when design wet bulb remains below the cold-water temperature.
  • Performance guarantees require applicable CTI/OEM test procedures, not this screening calculation.

How to interpret the result

Treat the primary result as an engineering screening value, then use the secondary outputs to understand scale, energy or operating impact. Confirm site measurements, equipment limits and applicable standards before making a design or control change.

Questions engineers ask

What is cooling-tower approach?

Approach is cold-water temperature minus entering-air wet-bulb temperature. A smaller positive approach means the leaving water is closer to the psychrometric limit, but achievable approach depends on tower selection, airflow, water distribution and load.

How is thermal effectiveness calculated here?

Effectiveness = range ÷ (hot-water temperature − current wet-bulb temperature), where range = hot water − cold water. It is a convenient screening ratio, not a substitute for CTI/OEM performance testing.

What does design wet-bulb margin show?

The owner compares current wet bulb with the entered design wet bulb and also shows what the approach would be at the same cold-water temperature. If design wet bulb is at or above that cold-water temperature, the scenario is physically inconsistent and should not be treated as valid performance.

Continue the calculation

Use the next calculator in the same engineering workflow instead of restarting from search.

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Methodology. MAXScanner shows the governing formula, entered values and calculation outputs so the estimate is auditable. The calculator is a screening aid; assumptions and equipment-specific limits remain part of the engineering review.