Why are fire protection units so mixed up?
On a single sprinkler project it is normal to work in three unit systems in one day: EN 12845 gives density in mm/min, NFPA 13 in gpm/ft²; the pump curve shows head in m or ft, the test report says bar or psi; and the sprinkler datasheet lists the K-factor both as L/min/bar½ and gpm/psi½. This converter brings together the ten quantities a fire protection engineer actually uses and converts the chosen unit into every unit in its category at once.
How to use it
- Pick the quantity (flow, pressure, density, K-factor, length, area, volume, temperature, power, velocity).
- Pick the unit you are converting from; the value box and slider work in that unit.
- The large tile shows the most common equivalent; the table lists every unit in the category with its definition.
The two dials show the same physical value on two different scales: the needles always sit at the same angle, only the numbers on the dial face change. For length, area and volume the dials become rulers, and for temperature they become thermometers.
Key conversion factors
| Quantity | Conversion | Note |
|---|---|---|
| Flow | 1 gpm = 3.78541 L/min · 1 m³/h = 16.6667 L/min | US gallon |
| Pressure | 1 bar = 14.5038 psi = 100 kPa = 10.1972 m H₂O | water at 4 °C |
| Density | 1 gpm/ft² = 40.7458 mm/min | mm/min = L/min/m² |
| K-factor | 1 gpm/psi½ = 14.4163 L/min/bar½ | K = Q/√P |
| Length | 1 ft = 0.3048 m · 1 in = 25.4 mm | exact by definition |
| Volume | 1 US gal = 3.78541 L · 1 UK gal = 4.54609 L | two different gallons |
| Power | 1 hp = 0.745700 kW | mechanical hp |
K-factor: why 14.4163?
The K-factor is flow divided by the square root of pressure. When you change units, both the numerator (flow) and the denominator (square root of pressure) change. Since 1 gpm = 3.78541 L/min and 1 bar = 14.5038 psi, the factor is 3.78541 × √14.5038 = 14.4163.
KSI = KUS × 14.4163 → K5.6 × 14.4163 ≈ 80.7 L/min/bar½Catalogue values are quoted rounded: K5.6 → K80, K8.0 → K115, K11.2 → K160, K14.0 → K200, K16.8 → K240, K25.2 → K360. In a hydraulic calculation it is better to use the manufacturer's stated value than the rounded nominal one.
Density: between mm/min and gpm/ft²
1 mm/min means one litre of water landing on one square metre every minute, so mm/min and L/min/m² are numerically identical. The NFPA 13 value of 0.10 gpm/ft² is about 4.07 mm/min, and 0.20 gpm/ft² is 8.15 mm/min. When comparing the two standards, convert the design area as well as the density: 1500 ft² ≈ 139.4 m².
Pressure and head
Head on a pump curve (m or ft) is expressed as a column of liquid. 1 bar ≈ 10.197 m of water and 1 psi ≈ 2.307 ft of water. The conversion assumes water at 4 °C (1000 kg/m³) and standard gravity; hot water is less dense, so the same head corresponds to slightly less pressure. The atmospheric difference of about 1.013 bar between gauge and absolute pressure is not removed by a unit conversion either: NPSH work uses absolute pressure, system pressures are normally gauge.
Temperature and temperature difference
Temperature scales have offsets: °F = °C × 9/5 + 32 and K = °C + 273.15. When converting a temperature difference, however, the offset does not apply: a 30 °C difference equals a 54 °F difference, not 86 °F. EN 12845 requires the sprinkler operating temperature to be at least 30 °C above the ambient; in Fahrenheit that margin is 54 °F.
Common mistake: in older British documents “gpm” may mean imperial gallons per minute. The UK gallon is 20% larger than the US gallon; converting a pump duty without noticing understates or overstates the capacity by about a fifth.
Accuracy and rounding
The calculator uses the constants that are exact by definition (1 ft = 0.3048 m, 1 in = 25.4 mm, 1 US gal = 3.785411784 L, 1 UK gal = 4.54609 L, 1 psi = 6894.757 Pa) at full precision; rounding happens only on display, to five significant figures. In hydraulic work, carrying intermediate results unrounded and rounding only the final figure avoids accumulated error, especially on long pipe runs.
The formula section shows, with the real numbers, how the chosen value is first converted to the category's base unit and then to the target unit. You can paste those lines straight into a calculation report.
Frequently Asked Questions
What is the SI K-factor of a K5.6 sprinkler?
K5.6 gpm/psi½ × 14.4163 ≈ 80.7 L/min/bar½, quoted in catalogues as K80. The factor is 3.78541 × √14.5038 because both the flow and the pressure units change.
How many bar is 100 psi?
1 psi = 0.0689476 bar, so 100 psi ≈ 6.895 bar. The same pressure is 689.5 kPa or about 70.3 m of water head.
What is 0.20 gpm/ft² in mm/min?
1 gpm/ft² = 40.7458 mm/min, so 0.20 gpm/ft² ≈ 8.15 mm/min. mm/min and L/min/m² are the same value.
Why is 32 not added when converting a temperature difference to °F?
The 32 offset applies to absolute temperatures only. A difference is simply multiplied by 1.8: a 30 °C difference is a 54 °F difference. In kelvin a difference equals the °C difference.
SprinkCalc — Fire Sprinkler Design in Three Standards
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