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Irrigation hose diameter and pressure loss
Updated 11 Oct 2026 · WaterTally
“1/2-inch or 3/4-inch hose?” is one of the most frequent questions on Italian gardening forums [8]. The answer matters more than it seems: water loses pressure to friction in a hose, and the loss grows roughly with the square of the flow and very quickly as the diameter gets smaller.
How much you lose in 25 metres
Calculation for water at 20 °C in a smooth 25 m hose, without fittings:
| Flow | 13 mm | 15 mm | 19 mm | 25 mm |
|---|---|---|---|---|
| 20 l/min | 15.2 m (1.5 bar) | 7.6 m (0.7 bar) | 2.4 m (0.2 bar) | 0.7 m (0.1 bar) |
| 30 l/min | 31.7 m (3.1 bar) | 15.7 m (1.5 bar) | 5.0 m (0.5 bar) | 1.3 m (0.1 bar) |
| 40 l/min | 53.8 m (5.3 bar) | 26.6 m (2.6 bar) | 8.4 m (0.8 bar) | 2.2 m (0.2 bar) |
Figures above the maximum head of a garden pump are not a mistake: they tell you that this flow, with this hose, cannot be reached. The pump will run at a lower flow, where its head is enough to overcome the hose and the height difference.
The diameter that counts is the internal one. Garden hoses sold as “1/2 inch” usually have an internal diameter of around 13 mm, but it varies from product to product: check the data sheet or measure it.
Table for 10 metres
The table below is generated by the same code as the calculator, for 10 m sections. For a 30 m hose, multiply by 3.
How we calculate it
- Loss: Darcy-Weisbach equation.
- Friction factor: Colebrook-White equation solved by iteration, starting from the Swamee and Jain formula [3].
- Water at 20 °C: kinematic viscosity 1.0035 × 10⁻⁶ m²/s [1].
- Roughness: 0.007 mm, the highest value given for smooth PVC and PE pipes [2]. An old hose, one with deposits inside or one that has been crushed, loses more.
- Excluded: quick connectors, tight bends, filters and valves add further losses; a narrow quick connector can cause as much loss as several metres of hose.
The calculation is verified with automated tests: in laminar flow it matches the Hagen-Poiseuille formula, in turbulent flow the result satisfies the Colebrook-White equation.
What to do in practice
- For irrigation with flows from 20 l/min upwards, a 13 mm hose is almost always the bottleneck: 19 mm or more is better, at least for the long section.
- Before replacing a pump because it “has no push”, try estimating the losses with the calculator: often the answer is the hose. For example, compare 25 m of 13 mm hose with the same system with 19 mm hose.
| Flow | 13 mm bore | 15 mm bore | 19 mm bore | 25 mm bore |
|---|---|---|---|---|
| 10 l/min | 1.75 m (0.17) | 0.88 m (0.09) | 0.28 m (0.03) | 0.08 m (0.01) |
| 20 l/min | 6.06 m (0.59) | 3.02 m (0.30) | 0.97 m (0.09) | 0.26 m (0.03) |
| 30 l/min | 12.68 m (1.24) | 6.30 m (0.62) | 2.00 m (0.20) | 0.53 m (0.05) |
| 40 l/min | 21.52 m (2.11) | 10.65 m (1.04) | 3.36 m (0.33) | 0.89 m (0.09) |
| 60 l/min | 45.74 m (4.49) | 22.49 m (2.21) | 7.03 m (0.69) | 1.85 m (0.18) |
| 80 l/min | 78.55 m (7.70) | 38.46 m (3.77) | 11.94 m (1.17) | 3.11 m (0.31) |
* transitional flow: uncertain value. Fittings and bends not included.
Sources
- Water - Dynamic (Absolute) and Kinematic Viscosity — The Engineering ToolBox, retrieved 11 Oct 2026
- Surface Roughness Coefficients — The Engineering ToolBox, retrieved 11 Oct 2026
- Explicit equations for pipe-flow problems — Swamee P. K., Jain A. K., Journal of the Hydraulics Division, ASCE, vol. 102, n. 5, pp. 657–664, maggio 1976 (scheda TRID), retrieved 11 Oct 2026
- Dubbio tubo 1/2 o 3/4 — Forum di Giardinaggio.it, discussione del 26 settembre 2021, retrieved 11 Oct 2026