How long should you run a sprinkler to water your lawn adequately?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

08 February 2026

Michael Kimani is worried that the grass in his compound will die because of the failure of the short rains last year. He is wondering how much water he needs for irrigation to keep the grass alive until the next rain season in April. He adds “I have … found many sources saying that grass needs 25mm of water every week … How do I convert that into litres?

Michael’s question brings out the reason why rainfall measurements are always given in millimetres and not in centimetres. We hear the weatherman reporting that there was 120mm of rain: why not just say 12cm? The answer is that the number of millimetres is equal to the number of litres of rainwater falling on one square metre of flat, horizontal ground.

This is easy to demonstrate… Volume is the base area multiplied by height; one square metre is 100cm x 100cm = 10,000sq.cm; 1mm = 0.1cm; thus, multiplying 10,000 by 0.1, we get 1,000 cubic centimetres (cc). Therefore, 1mm of rain means 1,000cc or 1L of water spread in one square metre of ground. QED.

So, if Michael measures the total area of his lawn in square metres, he will just need to multiply that number by 25mm to know the volume of water needed for irrigation every week. But that’s still not very useful because the water is not normally poured at one place from a bucket! It is spread with a sprinkler.

I asked Michael about his sprinkler and he said it sprinkles water in a circular pattern. So, I sent him to do some measurements: to find the radius of this circle and, second, to measure how long it takes to fetch one bucket (20L) of water from this tap. The answers were 2.5m and 2 minutes, respectively.

Now, a circle of 2.5m radius has an area of approximately 20sqm. If we fill this area with 25mm of water, the total volume will be 25mm x 20sqm = 500L. The tap delivers 20L in 2 minutes, so it will take about 50 minutes to give 500L.

Therefore, my advice to Michael is to run the sprinkler at one place for about one hour, then move it to the next location and repeat until the entire lawn is watered. I must stress on this: these calculations are only for Michael’s grass, not any other!