Turbulent case: two calculators, one number
A 50mm pipe with 2 m/s water flow gives a Reynolds number of exactly 100,000 from the pipe flow calculator. Running the same diameter, velocity, density, and viscosity through the pressure drop calculator — which needs Reynolds number internally to pick a friction factor formula — reports the identical 100,000.
Laminar case: the agreement holds, and it matters more here
A 20mm pipe with water crawling at 0.05 m/s gives a Reynolds number of exactly 1,000 from both calculators again. This time the agreement isn't just a nice consistency check — the pressure drop calculator uses this exact Reynolds number to compute its friction factor as 64 ÷ 1,000 = 0.064, confirmed directly against the real reported value. Any disagreement in the underlying Reynolds number would have silently thrown off this friction factor, and every downstream number built on it.
Why these agree despite being written independently
The two calculators don't call a shared Reynolds number function — each computes ρVD/μ on its own from the same four inputs. Getting identical results in both a laminar and a turbulent scenario confirms both implementations handle the formula (and its unit conversions, like diameter in millimeters converted to meters) the same way.
Why checking Reynolds number first is the right order of operations
Reynolds number decides everything that follows: whether flow is laminar or turbulent, and therefore which friction factor formula applies at all (a simple 64/Re, or the more involved Swamee-Jain approximation). Confirming a consistent Reynolds number between the flow-rate and pressure-drop calculations is the first thing worth checking before trusting either tool's downstream numbers for the same pipe.