GardenCalcs.uk
Water Pressure Calculator — Garden Irrigation Pressure & Flow
Calculate water pressure and flow rate for garden irrigation systems.
Where the water is fed from. Mains gives 1–4 bar; a gravity water butt only 0.1–0.3 bar (needs low-pressure drippers or a pump).
Internal bore of the supply pipe. 15 mm suits short runs; step up to 22 mm or 25 mm for long runs or many outlets to cut friction loss.
Total run from the source to the furthest outlet. Longer runs lose more pressure to friction.
Total drippers, sprinklers or taps fed by this run. Each fitting/tee adds a small pressure loss.
Positive = uphill, negative = downhill
Default 0.02 for typical garden pipe. Higher for older or rough pipes.
Default 3 bar. Check with your water supplier for actual value.
How We Calculate This
Pressure at the end of a pipe run is calculated by subtracting losses from the source pressure. Height loss uses the standard 0.0981 bar per metre of water column (1 mH₂O = 0.0981 bar).
Losses: Height loss = height × 0.098 bar/m. Friction loss = length × friction factor × (15 mm reference diameter ÷ actual diameter). Fitting loss = outlets × 0.05 bar.
The friction and flow figures are indicative estimates for sizing a garden system, not a precise engineering calculation — true pipe-friction modelling (Hazen-Williams or Darcy-Weisbach) depends on exact flow, bore and roughness. The maximum sprinkler count assumes a typical lawn/oscillating sprinkler drawing about 10 L/min; real demand ranges from 2–4 L/min for micro-sprinklers up to 15 L/min for a large oscillating sprinkler, so treat it as a guide.
Frequently Asked Questions
UK mains water pressure is usually between 1 and 4 bar, with 3 bar being a common average. Your water company can confirm your supply pressure, or use a simple gauge on an outside tap.
Yes. Friction losses increase with pipe length. A 50m run of 15mm pipe loses significantly more pressure than a 10m run. Larger pipe diameters reduce friction loss.
Yes, but gravity-fed water butts only provide about 0.1-0.3 bar. You need low-pressure drippers or a small pump. Standard sprinklers will not work from a water butt.
15mm is fine for short runs with few outlets. For longer runs or more outlets, step up to 22mm or 25mm to maintain adequate pressure and flow.
Every 1 metre of height difference adds or removes about 0.1 bar. Water flowing uphill loses pressure; downhill gains it. This matters especially for water butt gravity systems.
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Last updated: March 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.