Free hydraulics and pipe flow calculators
Quick, single-purpose calculators for everyday pipe and duct work: size a line, check a velocity, estimate a pressure drop or a heat loss in a few seconds. They are free, need no sign-in, and state the method and equations openly so you can check the working.
They are also not a separate calculator engine. Each page calls the same shipped solver components Fluid Network Studio runs, so the answer here is the answer the Studio gives for the same inputs, and it inherits the same published verification record - 31 cases, each with a named reference, its deviation and its tolerance.
What a single-topic calculator is for
Each page does one thing. It takes the handful of inputs one relation needs and evaluates it: no network to draw, no boundary conditions to set, nothing to save. That makes them the right tool for the checks that come up constantly on a live job, where opening a full model would cost more time than the question is worth:
- Sizing a single run, and seeing whether the velocity climbs past what you allow when you drop a diameter.
- Sanity-checking a pressure drop somebody else produced, before you act on it.
- Splitting an observed loss into its parts: friction along the pipe, static lift or a fitting.
- Producing an intermediate value another calculation needs, such as a Reynolds number, a friction factor or a loss coefficient.
- Checking a surface temperature or a heat loss on an insulated line before specifying lagging.
The common thread is that the flow is already known, or is fixed by one pipe and one head difference. Whenever that holds, a single relation is the whole answer and a calculator is genuinely enough.
When to build the network instead
The moment the flow split is itself part of the question, no single equation resolves it. A loop, a branch that rejoins, three reservoirs feeding one junction, or a pump settling onto its own duty point against the system all have flows that depend on one another, so they have to be solved together. Elevation makes this worse rather than better: the lowest pressure in a branched main is often not at the point furthest from the source, and only a solve tells you which branch it is.
That is what the Studio does, with the same friction models these calculators use, plus pumps and fans on their curves, compressible gas, heat transfer along the line and non-Newtonian fluids. The worked examples are the fastest way to see the difference: each one is a real network with its solved result published, free to open and re-solve.
The calculators
Pipe flow and pressure drop
Head loss and pressure drop from flow, or flow from an available head (Darcy-Weisbach with Colebrook-White friction, evaluated by the explicit Churchill correlation).
Pipe heat loss and surface temperature
Total heat loss, temperature drop and the temperature of every insulation layer, including the outer surface for safe-touch checks.
Reynolds number
Laminar, transitional or turbulent.
Darcy friction factor
From Reynolds number and relative roughness, by the explicit Churchill correlation that reproduces the Colebrook-White curve.
Pipe velocity
Mean velocity from flow and diameter, or the reverse.
Pump power
Hydraulic, shaft and electrical power for a duty.
Hazen-Williams pressure drop
The empirical head-loss method for water.
Orifice plate loss coefficient
The irreversible loss coefficient K and pressure drop of a sharp-edged orifice.
Rectangular duct pressure drop
Friction pressure drop of a non-circular duct via the hydraulic diameter.
Ready for a full network? Open the Studio to connect pipes, pumps, fans and boundary conditions and solve the whole system at once, with liquids, gases and heat transfer.
Frequently asked questions
Are these calculators free?
Yes, and there is nothing to sign up for. Every calculator on this page runs in your browser with no account and no sign-in. The Studio has a free tier too: it solves your own liquid networks without an account, and the built-in example networks are always free to open and run. The paid plan is for compressible gas, heat transfer, non-Newtonian fluids, larger networks, CFD coupling and the result exports, none of which these calculator pages touch.
How accurate are they?
These pages are not a second implementation of the equations. They call the shipped solver components directly, so the number a calculator gives is the number the Studio gives for the same inputs. That solver is published case by case: across the 31 verification cases, the closed-form and OWA-EPANET 2.2 checks agree with their references to within 0.32%, and the independent NASA GFSSP and published-element cross-checks agree to within 2.61%. Every case is re-solved by the real solver when the site is built and the build fails if any value leaves its acceptance band, so those bounds describe the code serving this page rather than a past run. They bound the method, not your inputs. Roughness, internal diameter and fluid properties at the working temperature are still yours to get right, and they are where most real error comes from.
What units can I use?
Both metric and imperial. Every calculator carries a Metric (SI) / Imperial (US) switch, and flipping it converts the values already in the fields rather than clearing them, so you can enter a diameter in inches and read a pressure drop in psi. Internally every calculation runs in SI and converts only for display, which is the same unit boundary the Studio uses.
Which equations do they use?
Pipe friction is Darcy-Weisbach with the Churchill (1977) friction factor, one explicit expression that reproduces the implicit Colebrook-White equation (the relationship the Moody chart plots) to within about two per cent across laminar, transitional and turbulent flow, with no iteration. Hazen-Williams is available where it is the convention for water, in its SI form hf = 10.674 L Q^1.852 / (C^1.852 D^4.871). The orifice and rectangular-duct pages call the solver's own orifice and hydraulic-diameter models rather than a restatement of them. Each calculator page states its own method and shows a worked example with the numbers filled in, so you can check the working rather than trust the box.
When should I build a network instead of using a calculator?
As soon as the flow split is part of the question. A calculator evaluates one relation for one pipe, which works when the flow is already known or is fixed by a single head difference. A loop, a branch that rejoins, two reservoirs feeding one junction, or a pump finding its own duty point against the system all have flows that depend on each other, and no single equation resolves them. That needs the whole network solved at once, which is what the Studio does. Building and solving the built-in example networks is free.
Can I rely on these numbers for design?
They are analysis, not a design decision. Like any analysis tool, a calculator supports a qualified engineer's judgement rather than replacing it, and a result should be reviewed before it is relied upon. What these pages give you is a result you can check: the method is stated, the worked example is reproducible, and the solver behind it is published with its references and deviations.
Every case behind the accuracy figures above, with its reference source, reference value, Studio value, deviation and acceptance tolerance, is listed on the verification page.