<p>Rainfall rate as mm/hr or in/hr is in terms of length, not volume.  You're thinking of it as a &quot;per unit area&quot; rate, but that would be reported in volume or mass, not length.  Another way to put it is that the volume-per-area is already implicitly solved by reporting in units of length.</p>
<p>Imagine using a physical rain gauge with depth markers.  If you convert from mm to inches, does the area occupied by the gauge change?</p>
<p>Note that your mm/m2 calculation would have rainfall being measured in units of inverse length and inverse time, which doesn't make physical sense.</p>

CC BY-SA 4.0
2024-09-04T16:47:34.837Z
2024-09-04T16:47:34.837Z
damp_civil
<p>when converting the Rainfall Rate unit from mm per HR (Sq. m) to  inch per HR (Sq. ft)
why are they multiplying by 25.4 only and not considering the conversion from m2 to ft2</p>
<hr />
<p>for example</p>
<p>mm/m2 --&gt; in/ft2 = (1/25.4 in)/(10.764 ft2) = 0.0036 in/ft2</p>

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2024-09-04T15:37:38.943Z
Who are "they"? You should provide more context to the question. Welcome to Earth Science SE!
Jean-Marie Prival
<meteorology><weather-forecasting><rainfall><storms>
Rainfall Rate - unit conversion
a. m.
# Rainfall Rate - unit conversion

(Asked by: username_0 on 2024-09-04)

when converting the Rainfall Rate unit from mm per HR (Sq. m) to inch per HR (Sq. ft)
why are they multiplying by 25.4 only and not considering the conversion from m2 to ft2

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for example

mm/m2 --> in/ft2 = (1/25.4 in)/(10.764 ft2) = 0.0036 in/ft2

**Tags:** meteorology, weather-forecasting, rainfall, storms

> **Comments:**
> **username_1**: Who are "they"? You should provide more context to the question. Welcome to Earth Science SE!  

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## 1 answers

### Answer by username_2 on 2024-09-04. Score: 1 | Accepted answer

Rainfall rate as mm/hr or in/hr is in terms of length, not volume. You're thinking of it as a "per unit area" rate, but that would be reported in volume or mass, not length. Another way to put it is that the volume-per-area is already implicitly solved by reporting in units of length.

Imagine using a physical rain gauge with depth markers. If you convert from mm to inches, does the area occupied by the gauge change?

Note that your mm/m2 calculation would have rainfall being measured in units of inverse length and inverse time, which doesn't make physical sense.


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