How Much Water Does My Garden Need Each Week After Rain

How much water does my garden need each week after rain? Most gardens need about 25 mm of water a week in the growing season, from rain and watering together. If 10 mm of rain fell, the shortfall is 15 mm, so a 15 m2 bed needs 225 litres, or 300 litres with a 75 percent efficient sprinkler.

The depth alone does not answer the question, because a small bed and a large garden can both need 25 mm a week, and the large garden still needs far more water. This blog is a calculation, not general advice. It uses the weekly need, the rain that fell, the area, and the method efficiency to give a number you can act on.

How much water does my garden need by bed area

Start with three numbers. First, the weekly need, which is 25 mm here. Second, the rain that fell this week, measured with a rain gauge or read from a weather app. Third, the area of the garden you water. The shortfall is the weekly need minus the rain, and it can never fall below zero. If rain meets or beats the weekly need, the shortfall is zero and no watering is needed that week.

Once you have the shortfall, the volume follows from a unit fact on the calculator page. One mm of water over 1 m2 is exactly 1 litre. So a shortfall of 10 mm over a garden of 10 m2 is 100 litres. Method efficiency changes the volume you must apply, because some water is lost to evaporation and runoff. The formula is water equals shortfall times area divided by method efficiency. The only verified efficiency figure on the page is 75 percent for a sprinkler, so this blog shows the unadjusted figure at 100 percent beside the sprinkler figure at 75 percent. For any other method, enter your method efficiency in the Garden Watering Schedule Calculator and read the result there.

The 25 mm rule in plain numbers

The 25 mm rule means 25 litres per square metre per week, from rain and watering together. A garden of 5 m2 needs 125 litres in a week with no rain. A garden of 10 m2 needs 250 litres. A garden of 30 m2 needs 750 litres. A garden of 50 m2 needs 1,250 litres. Those are the unadjusted figures, at 100 percent, with no rain deducted. They rise in direct proportion to area.

The same rule in imperial terms is 1 inch a week, which is the rounded equivalent of 25 mm. One inch over 1 sq ft is 0.62 US gallons, so 1 inch over 100 sq ft is 62 US gallons. Keep the metric and imperial examples separate, and treat the imperial figures here as rounded equivalents, not exact conversions. Rain is part of the weekly total, not an extra on top of it. If 5 mm fell, you add only the remaining 20 mm. If 25 mm fell, you add nothing.

Worked example: a 15 m2 bed after 10 mm of rain

Take a bed of 15 m2 in a week when 10 mm of rain fell. The weekly need is 25 mm, so the shortfall is 15 mm. The unadjusted water is 15 times 15, which is 225 litres. With a 75 percent efficient sprinkler, the water to apply is 225 divided by 0.75, which is 300 litres.

The page worked example uses the same method on a different bed: a 20 m2 vegetable bed needing 25 mm a week, after 5 mm of rain, needs 20 mm more, giving 533 litres with a 75 percent efficient sprinkler.

Litres by bed area with no rain

The table below gives the weekly water for six bed areas when no rain fell, so the shortfall is the full 25 mm. Rainfall figures in tables are calculator inputs, not claims about any real place.

Bed area	Shortfall with no rain	Litres at 100 percent	Litres with 75 percent sprinkler
5 m2	25 mm	125 L	167 L
10 m2	25 mm	250 L	333 L
15 m2	25 mm	375 L	500 L
30 m2	25 mm	750 L	1000 L
40 m2	25 mm	1000 L	1333 L
50 m2	25 mm	1250 L	1667 L

The sprinkler column is always one third higher than the unadjusted column, because dividing by 0.75 is the same as multiplying by 1.333. A 10 m2 bed needs 250 litres with no rain, and 333 litres through a sprinkler. A bed of 20 m2 needs 500 litres unadjusted and 667 litres through a sprinkler.

Shortfall by rain for a 30 m2 garden

The table below fixes the area at 30 m2 and varies the rain. Rainfall figures here are calculator inputs, not claims about any real place.

Rain this week	Shortfall	Litres at 100 percent	Litres with 75 percent sprinkler
0 mm	25 mm	750 L	1000 L
5 mm	20 mm	600 L	800 L
10 mm	15 mm	450 L	600 L
15 mm	10 mm	300 L	400 L
20 mm	5 mm	150 L	200 L
25 mm	0 mm	0 L	0 L
30 mm	0 mm	0 L	0 L

Every 5 mm of rain on a 30 m2 garden saves 150 litres unadjusted, and it saves 200 litres through a sprinkler. A week with 5 mm of rain needs 600 litres unadjusted, and a week with 20 mm needs only 150 litres. The last two rows show a zero in both water columns, so no watering is needed.

Worked example in gallons: a 200 sq ft bed after half an inch of rain

Work the same logic in imperial units, on its own, without converting the metric example. Take a bed of 200 sq ft in a week when half an inch of rain fell. The weekly need is 1 inch, so the shortfall is 0.5 inch. The water is 0.5 times 200 times 0.62, which is 62 US gallons. It is not an exact conversion of the 15 m2 metric example above, because the areas and the rain amounts differ. With a sprinkler, divide 62 by 0.75 to get 83 US gallons.

Gallons by bed area and rain

The table below gives US gallons for five bed areas and five rain amounts, all at 100 percent, with the weekly need of 1 inch. Rainfall figures here are calculator inputs, not claims about any real place, and the gallon figures are rounded to the nearest gallon.

Bed area	0 in rain	0.25 in rain	0.5 in rain	0.75 in rain	1 in rain
50 sq ft	31 gal	23 gal	16 gal	8 gal	0 gal
100 sq ft	62 gal	47 gal	31 gal	16 gal	0 gal
200 sq ft	124 gal	93 gal	62 gal	31 gal	0 gal
300 sq ft	186 gal	140 gal	93 gal	47 gal	0 gal
500 sq ft	310 gal	233 gal	155 gal	78 gal	0 gal

Check one cell by hand. A bed of 300 sq ft with 0.25 inch of rain has a shortfall of 0.75 inch. The water is 0.75 times 300 times 0.62, which is 139.5 gallons, rounded to 140 gallons, which matches the table. The last column is zero for every area, because 1 inch of rain meets the 1 inch weekly need.

What the sprinkler figure changes

The sprinkler figure changes only the volume you apply, not the depth the soil needs. At 75 percent efficiency, the applied volume is the unadjusted volume divided by 0.75, which is one third higher in every case. A bed that needs 225 litres unadjusted needs 300 litres through a sprinkler.

Do not apply the 75 percent figure to a method it does not describe. The page gives 75 percent for a sprinkler only, and it states that efficiency figures are typical. If you water by another method, use the unadjusted figure as the amount the soil needs, enter your method efficiency in the calculator, and read the applied volume there. The Drip Irrigation Planner can help you plan the layout once you know the weekly volume.

Splitting the weekly total across sessions

The weekly total is not meant to fall in one watering. The calculator gives sessions by soil: 3 for sand, 2 for loam, and 1 for clay. Deep watering a few times a week sends roots down where the soil stays moist. The page FAQ gives the same guidance: usually two or three deep waterings a week in summer, more often on sandy soil, less on clay.

Take the metric worked example and split it by soil. The 15 m2 bed after 10 mm of rain needs 300 litres through a sprinkler. On loam, with 2 sessions, each session is 150 litres. On sand, with 3 sessions, each session is 100 litres. On clay, with 1 session, the whole 300 litres falls in one session. At 12 litres per minute, from a 10 litre bucket filled in 50 seconds, a 150 litre loam session takes about 13 minutes.

How to measure rain and area

Measure rain with a rain gauge where you can, because a gauge gives the depth that fell on your garden. A weather app gives a useful reading too, and the calculator page names both as sources for the weekly rain figure. Use the total for the week, not the rain from a single shower.

Measure area by splitting the garden into simple shapes and adding them up. The Garden Area Calculator takes rectangles, circles, triangles and ovals, and it gives the total in square metres and square feet. Measure only the ground you water, and leave out paths, patios, and buildings.

When rain covers the whole week

When rain meets or beats the weekly need, the answer is zero, and the right action is to water nothing that week. That case is easy to miss, because a fixed watering routine keeps running through wet weeks unless someone checks the rain first. The tables make the zero visible, with a full row of zeros at 25 mm of rain in the metric table and a full column of zeros at 1 inch of rain in the imperial table. Those zeros save the whole weekly volume, which on a 50 m2 garden is 1,250 litres unadjusted.

A wet week does not mean the next week needs nothing. The shortfall is worked week by week, so a week with 30 mm of rain gives a zero, and the next week with no rain gives the full weekly volume again. Do not carry a surplus from a wet week into the next week as a credit.

Common mistakes

The first mistake is watering the full 25 mm without deducting rain. A garden that had 10 mm of rain needs only 15 mm more, and adding the full depth wastes 150 litres on a 15 m2 bed and 500 litres on a 50 m2 garden. The second mistake is using depth alone and forgetting area. A depth of 25 mm is 125 litres on a 5 m2 bed and 1,250 litres on a 50 m2 garden.

The third mistake is mixing metric and imperial figures in one calculation. Pick one system and keep the units matched through the whole week. The fourth mistake is applying the 75 percent sprinkler figure to every method. That figure belongs to a sprinkler only. The fifth mistake is treating a zero shortfall as a reason to water a little anyway. A zero means the weekly need is met. The sixth mistake is forgetting that containers and new plantings need more than this guide, as the page states.

How to use the calculator

Open the Garden Watering Schedule Calculator and enter the area you water. Set the weekly need to 25 mm, or 1 inch in imperial units, and enter the rain that fell this week from your gauge or weather app. Choose your soil, so the calculator gives the session count for sand, loam, or clay, and choose your watering method, so it can apply the efficiency you enter. Time your hose with a bucket of known size if you need the flow, using the page method of a 10 litre bucket in 50 seconds for 12 litres per minute, then water for the minutes shown.

If you collect rain for the garden, the Rain Barrel Calculator shows how much water a roof can supply in a storm, and the Rainwater Harvesting Calculator shows how much a roof can supply in a year, so you can see whether stored rain can cover the shortfall here.

Frequently asked questions

Q: How much water does my garden need each week with no rain?
A: About 25 mm a week, which is 25 litres per square metre. A 10 m2 garden needs 250 litres, and a 50 m2 garden needs 1,250 litres.

Q: How much water does my garden need after 10 mm of rain?
A: The shortfall is 15 mm. A 15 m2 bed needs 225 litres at 100 percent, or 300 litres with a 75 percent efficient sprinkler.

Q: Do I need to water if 25 mm of rain fell this week?
A: No. The shortfall is zero when rain meets the weekly need, so no watering is needed that week. If more than 25 mm fell, the shortfall stays at zero.

Q: How many gallons does a 200 sq ft garden need each week?
A: With no rain, 124 US gallons at 100 percent. After half an inch of rain, the need is 62 US gallons.

Q: How often should I water my garden in summer?
A: Usually two or three deep waterings a week in summer, more often on sandy soil, less on clay.

Q: Is morning the best time to water?
A: Yes. Morning is best, because there is less evaporation, and leaves dry quickly, which reduces disease.