Type in a rain gauge reading to change it between points, millimetres and inches, or to see how many litres it puts in your water tank. One point is 0.254 mm, 100 points is one inch (25.4 mm), and 1 mm of rain on 1 square metre of roof is 1 litre.
Points, mm And Inches
25 mm = 98.4 points = 0.98 inches
Rain To Tank Litres
Every millimetre of rain on one square metre of roof is one litre of water. So litres = rain in mm × roof area in square metres. Measure the roof from above, length × width including the eaves, not along the slope.
25 mm on 150 m² = 3,750 litres
That's the most the roof can catch. A bit less reaches the tank: some splashes off or overflows the gutters in heavy rain, and a first-flush diverter keeps the first few litres out.
Points To mm Table
| Points | mm | Inches |
|---|---|---|
| 5 | 1.3 | 0.05 |
| 10 | 2.5 | 0.10 |
| 15 | 3.8 | 0.15 |
| 20 | 5.1 | 0.20 |
| 25 | 6.3 | 0.25 |
| 30 | 7.6 | 0.30 |
| 40 | 10.2 | 0.40 |
| 50 | 12.7 | 0.50 |
| 60 | 15.2 | 0.60 |
| 70 | 17.8 | 0.70 |
| 75 | 19.1 | 0.75 |
| 80 | 20.3 | 0.80 |
| 90 | 22.9 | 0.90 |
| 100 | 25.4 | 1.00 |
| 125 | 31.8 | 1.25 |
| 150 | 38.1 | 1.50 |
| 200 | 50.8 | 2.00 |
| 250 | 63.5 | 2.50 |
| 300 | 76.2 | 3.00 |
| 400 | 101.6 | 4.00 |
| 500 | 127.0 | 5.00 |
How The Conversions Work
- A point is one hundredth of an inch, so points ÷ 100 = inches.
- An inch is 25.4 mm, so points × 0.254 = mm, and mm ÷ 0.254 = points.
- Quick rule: divide points by four for roughly the millimetres (100 points is about 25 mm).
Australia switched rainfall to millimetres in the 1970s, so older rain books are often in points or inches. Our guide to points of rain to mm explains more, and how to read a rain gauge covers getting an accurate reading in the first place.
Keep Your Readings In Any Unit
The free GaugeMate rain gauge app lets you switch between millimetres, points and inches at any time, and every total, chart and report follows along, so old and new records line up without converting by hand.