Water Pipe Sizing Calculator
Bore from flow and velocity, with actual velocity and friction loss.
Quick answer
With the default inputs: Design flow 1.5 L/s · Economic velocity 1.2 m/s · Pipe length 100 etc.Calculated bore 39.9 mm
Edit the inputs below to recalculate.Inputs
Result
Calculated bore
39.9mm
| Suggested DN | 40mm |
| Actual velocity | 1.19m/s |
| Design flow | 1.50L/s |
| Per hour | 5.40m³/h |
| Required area | 1250mm² |
| Selected area | 1257mm² |
| Friction loss | 3.63m |
| Minor loss | 1.09m |
| Total loss | 4.72m |
| Loss per 100 m | 3.63m |
Sizing uses the bore, while DN is only nominal: a DN50 galvanised pipe is about 53 mm inside, but DN50 PPR is only 40-44 mm because of its wall, which raises velocity by over 30%. Size plastic pipe on bore, not DN, or friction loss is badly underestimated. Friction uses Darcy with λ taken as 0.02; exact values come from hydraulic tables.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Area | Q ÷ v, with Q in m³/s |
| Bore | √(4A ÷ π) |
| Actual velocity | Q ÷ (π d²/4) |
| Friction | λ × (L ÷ d) × (v² ÷ 2g) |
| Minor losses | friction × share |
| Per 100 m | loss ÷ length × 100 |
1.5 L/s at 1.2 m/s: 1250 mm² required, a 39.9 mm bore, so DN40 at an actual 1.19 m/s — 3.61 m of friction over 100 m and 4.69 m including 30% minor losses.
How to work it out
- Convert the design flow to m³/s: 1 L/s is 0.001 m³/s. Getting this wrong puts the bore out by a factor of 30.
- Set the economic velocity: 0.6-1.0 m/s for DN15-20 domestic supply, 0.8-1.2 for DN25-40, 1.2-1.5 above DN50, and up to 2.5 m/s for fire mains. The economic band is where pipe cost plus pumping energy is lowest.
- Compute the required area A = Q ÷ v, then the theoretical bore d = √(4A ÷ π).
- Round up to the next standard size. PPR and PVC pipe is designated by outside diameter (De) while DN is nominal: a De25 PPR pipe in S2.5 has an internal bore of only about 18 mm.
- Check the actual velocity v = Q ÷ (π d²/4). Too high means noise and water hammer, too low means wasted material.
- Work out friction loss h = λ × (L÷d) × (v²÷2g) and add minor losses at 20-40% of friction. Express it as 1000i per 100 m to check against pump head and mains pressure.
Where it goes wrong
- Pushing velocity to 2.5 m/s to save a pipe size. Above 1.5 m/s domestic supply becomes audibly noisy and water hammer appears, worst at night, and in tall buildings it shakes brackets loose.
- Treating the PPR outside diameter as the bore. Sizing De25 as 25 mm when the bore is 18 mm raises velocity by 90% and head loss by a factor of 3.4, so terminal pressure never matches the design.
- Ignoring minor losses. A single 90° elbow is worth 1-2 m of straight pipe, and in plant rooms and riser cupboards they can account for half the total loss.
- Calculating friction but not static head. In downfeed or roof-tank systems the elevation difference dominates and friction alone tells you nothing.
Material and cost
PPR pipe runs 6-10 CNY/m at De25, 10-16 at De32 and 16-25 at De40, with fusion fittings at 3-20 CNY each. Moving from De25 to De32 adds about 60% to material but cuts pump head by 4-6 m, which pays back in running cost. Pipe only — insulation, supports and labour excluded.
On site
- Domestic supply and fire mains have different velocity limits; a combined system takes the stricter one.
- Tall buildings need pressure zoning to keep static pressure under 0.45 MPa per zone, otherwise ground-floor taps blast and meters fail early.
- Insulation changes the outside diameter, which affects sleeve sizing and ceiling voids. Choose the pipe size with insulation thickness already in mind.
- Quick field estimate of available flow: a fully open DN15 tap delivers about 0.15 L/s, DN20 about 0.25, DN25 about 0.4 — time a bucket and you have the answer.
Basis
- GB 50015-2019 Standard for design of building water supply and drainage, hydraulic calculation of pipes
- GB 50242-2002 Code for acceptance of construction quality of building water supply, drainage and heating works
- CJJ/T 98-2014 Technical specification for building water supply plastic piping engineering
FAQ
Why keep velocity between 1.0 and 1.5 m/s?
It balances capital and comfort. Lower velocity means a bigger, dearer pipe; higher velocity raises friction loss with the square of velocity, pushing up pump energy, adding noise and erosion, and inviting water hammer. Fire mains allow 2.5 m/s because they only run during a fire, so long-term energy does not matter.
Why can't DN be used for PPR?
Because plastic pipe is designated on outside diameter and has much thicker walls. DN25 PPR is only 16.6-18.4 mm inside against 26.9 mm for DN25 galvanised — 40% smaller. Choose DN25 for both and the plastic line sees over twice the velocity and loss, which shows up as poor pressure at the top of a building. Size on bore, or on velocity.
What is water hammer and how is it avoided?
It is the pressure wave that runs back and forth when a valve shuts suddenly and the flow stops, converting kinetic energy into a surge up to three to five times working pressure — enough to split pipe and wreck valves. Prevent it with slow-closing valves or surge arrestors, velocities under 1.5 m/s, air release valves at high points, and non-return valves with bypass cushioning at pump outlets.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Water Pipe Sizing Calculator. 2026-10-25. https://jianzhujisuan.com/en/pipe-diameter
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.