Rainwater harvesting calculator.

One millimeter of rain on one square metre of roof equals one litre of water run-off

To calculate the approximate amount of rainwater you are likely to harvest, use the following formula:

Estimated Net Runoff from an Impervious Catchment Surface Adjusted by its Runoff Coefficient

catchment area (m2) x rainfall (mm) x runoff coefficient = net runoff (litres)

Run-off (litres) = A x (Rainfall - B) x Roof Area

A is the efficiency of collection and values of 0.8-0.85 (i.e. 80-85% efficiency) have been used (eg Martin, 1980).

B is the loss associated with adsorption and wetting of surfaces and a value of 2 mm
per month
(24 mm per year) has been used (eg Martin, 1980).

Rainfall should be expressed in mm and Roof Area in square metres (m2)


In an area receiving 600 millimetres of rain a year with a rooftop catchment surface that is 11 metres long and 4.55 metres wide, and you want to know how much rainfall can realistically be collected off that roof in an average year. You want a conservative estimate of annual net runoff, so you use a runoff coefficient of 80% or 0.80.

catchment area (m2) = length (m) x width (m)

2 x (length (m) x width (m)) x rainfall (mm) x 0.80 = net runoff (litres)
2 x (11 m x 4.55 m) x 600mm x 0.80 = net runoff (litres)

100m2 x 600mm x 0.80 = 48,112 litres

48,112 litres = net runoff

A realistic estimate of the volume of water that could be collected off this 11 meter by 4.55 meter roof in a year of average rainfall is 48,112 litres.

A 1-acre self-sufficient small-holding


We’re shopping for land in Hungary and while prices are relatively low, when compared to those in Scotland, it’s possible that our budget will only stretch to a 1-, or 2-acre plot. Ideally we’re looking for 4 or 5 acres minimum however 1 or 2 acres will be suitable.

With this in mind I’m collecting information and learning how to “survive” on smaller plots.

Download: Start-a-1-acre-self-sufficient-homestead

Automated Rain Barrel Irrigation Project

Something to read later …

Irrigating a small garden is a relatively relaxing and sometimes therapeutic activity (well if you are a botany/biology nerd [or really in need of a hobby]) but going away for any length of time can present a problem. The simple solution of course is to purchase a hose-bib/timer package from Home Depot for about $30 and set up a small drip system to efficiently water each plant or row of plants (rather than a wasteful sprinkler system). Going along with conserving water is using a rain barrel collection system to capture rain for later use thus eliminating or more likely reducing treated water consumption.

Automated Rain Barrel Irrigation Project. via hAxOrYoUrMoThErHaRdRiVe.