DP Flow Square Root Calculator
By Santaji GadepatilinUpdated October 27, 2026
Flow rate (m³/h) vs Measured DP (mbar) — other inputs held at your current values.
How to use this calculator
Enter the transmitter’s DP range (DP at maximum flow) and the maximum flow. Then type the measured DP — or a percentage — to get the actual flow. You can also work backwards: type a flow to see the DP it should produce.
The formula
Flow % = √(DP %) Q = Qmax × √(ΔP ÷ ΔPmax)
Worked example
Range 0–250 mbar = 0–100 m³/h. Measured DP = 62.5 mbar.
- DP % = 62.5 ÷ 250 = 25%
- Flow % = √0.25 = 50%
- Flow = 50 m³/h
DP vs flow table
| DP % | Flow % |
|---|---|
| 1% | 10% |
| 4% | 20% |
| 9% | 30% |
| 25% | 50% |
| 49% | 70% |
| 81% | 90% |
| 100% | 100% |
At 10% flow the transmitter sees only 1% of its range, which is why turndown is limited.
With a 4–20 mA signal
If the transmitter output is linear with DP, mA = 4 + 16 × DP%. For 50% flow (25% DP) that is 8 mA, not 12 mA. If the transmitter extracts the square root itself, 50% flow reads 12 mA. Use the 4–20 mA calculator for the conversion.
Frequently asked questions
Why do DP flow meters need square root extraction?
Flow through an orifice or venturi is proportional to the square root of the differential pressure. Without extraction, a reading of 25% DP would be wrongly shown as 25% flow when it is really 50%.
Where is the square root applied — transmitter or control system?
Either, but never both. Applying it twice is a common commissioning error that makes readings far too high at low flow.
What is low-flow cut-off?
Near zero the square-root curve is very steep, so tiny DP noise causes large flow swings. Most systems force flow to zero below about 1–6% DP.