Your web browser (Internet Explorer) is out of date. Some things will not look right and things might not work properly. Please download an up-to-date and free browser from here.

Rainfall is measured in mm not mL

With all the rainfall figures coming in from the recent floods a theme was noticed – many of you measure rainfall in millilitres not millimetres.

We thought we’d go straight to the experts to get an explanation. Who better than former MetService Weather Ambassador Bob McDavitt.

Bob says rain is measured in a rain gauge its depth is measured in millimetres (mm).

“A mm of rain spread out over a square metre weighs around 1Kg and takes up 1 litre in volume. if we spread the same depth of rain over a hectare (100m x 100m) then it weighs a tonne / 1000kg”

So 1 mm of rain = 1L per square M.

“Millilitres of rain is probably what hydrologists and roof builders work with” says Bob.  “If you know the square metres of your roof (when reduced to a flat surface) then multiply that by the mm of rain to get the litres going into your rain storage tank, so if your place got 50mm of rain last night (two inches) and your (flattened) roof is 50 sq metres then that is 50 x 50 = 2,500 litres into the storage tank, or a quarter of a 10,000 litre tank”. 

As to how some people switched from mm to ml? Bob has his theory: “I think that’s happened because the slang term for millimetres of rain is mils of rain, and it is so easy to from mils to ml.”

The correct measurement for rainfall is in millimetres (mm).




LjAWgTn on 9/05/2022 4:11pm

Hi Bob McDavitt. My partner and I lived above you in the 9o’s in H Rd, HB. We looked after your mail when you went on the Whitbread RtWR! (a great neighbour and a good guy a man on top of his game is Bob McD)
You won’t remember us, we weren’t there that long, 6-12 months, but then again you might ha, but after your name popping up I wanted to attempt to send you kind regards and best wishes for the future. from Lisa

andy2000 on 4/11/2019 11:11pm

Hiya, I too wonder if you could correct the rate per hectare…it should read 10,000L per hectare.

rajib nazark on 17/09/2020 9:17am

MY/SG detected in NZ

Scott on 29/09/2019 6:02pm

Conical, glass full scale marked one liter. At the bottom marked off in mL.

I was looking for a chart to aid in converting this to inches so we could use this old gauge.

WW Forecast Team on 29/09/2019 6:55pm

A simple google search should bring up plenty of charts converting mL to inches.

Good luck! 🙂


MArtyn on 31/07/2018 11:43pm

As per previous comments, are you going to correct the math on this article

1mm rain equals 10,000 litres (10t, 10m3) per hectare, not 1,000 litres.


Darryl J. Bosch on 3/07/2017 9:48am

This is not true : SO 1 mm of rain = 1 ml per square m

It should read : SO 1 mm of rain = 1L per square m

Please correct your site as I was going to use this article for my Yr8 maths class to reference, but I am unable to do so.


WW Forecast Team on 3/07/2017 7:47pm

Hi there, Bob’s typo has been fixed.



View more comments

Related Articles