Heat Exchanger LMTD Calculator
Enter hot- and cold-side inlet and outlet temperatures, then choose parallel or counter flow. Optionally calculate area from heat duty and overall U.
A QUICK WALKTHROUGH
How to use this tool
- Enter four terminal temperatures in one temperature unit and select the flow arrangement.
- Review both positive terminal differences and the log mean temperature difference.
- Optionally enter heat duty and overall heat-transfer coefficient to estimate area.
Terminal pairing
Parallel flow pairs hot inlet with cold inlet and hot outlet with cold outlet. Counter-flow pairs hot inlet with cold outlet and hot outlet with cold inlet. Both terminal differences must be positive; LMTD uses their logarithmic mean.
Optional area estimate
When duty Q and overall coefficient U are entered, area A = Q/(U × LMTD). Use Q in W and U in W/(m²·K) for area in m², or keep any consistent unit set.
Single-pass scope
This is the ideal single-pass LMTD calculation. It does not calculate correction factors for multipass or cross-flow exchangers, exchanger effectiveness, pressure drop, fouling, or design suitability. Formula reference: ASHRAE Handbook—Fundamentals, Chapter 4, Mean Temperature Difference Analysis: https://handbook.ashrae.org/Handbooks/F25/SI/f25_ch04/f25_ch04_si.aspx .
GOOD TO KNOW
Common questions
Does this handle multipass exchangers?
No. It reports the ideal single-pass LMTD and does not apply an F correction factor.
Can the terminal differences be zero or negative?
No. A non-positive terminal difference has no valid real logarithmic mean for this calculation.
Are inputs uploaded?
No. The calculation runs in this browser.