A small roof collects a surprising amount of rain. One inch of rain on 250 square feet of roof can fill two 55-gallon barrels. Enter the roof area that drains to your downspout, the rainfall, and your barrels, and this calculator shows how much you catch and how many days it waters your tomatoes.
Updated September 26, 2026 · Reviewed by Aiza Anwar
Try a preset (6 in-ground plants, warm weather):
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What Your Result Means: the catch is what your roof sheds after typical losses. Your barrels keep only what fits, and the rest overflows. The days figure is how long the water you actually stored (up to your barrels’ capacity) waters your tomatoes with no more rain.
How Much Rain Can a Rain Barrel Catch?
Each inch of rain on each square foot of roof is about 0.62 gallons, and a roof sheds most of it. Enter the roof area feeding your downspout, the rainfall, and your barrels to see the catch and how long it waters your plants.
- Enter the Roof Area, only the part that drains to this downspout.
- Enter the Rainfall and your barrel size and count.
- Add Your Tomatoes, plants, setup, and weather, to see how long the water lasts.
What Each Result Means
| Result | What It Tells You |
|---|---|
| Rain collected | Gallons your roof sheds after losses |
| Barrels filled | How full your storage gets, and any overflow |
| Rain to fill | Inches of rain needed to top your barrels up |
| Days of watering | How long the stored water lasts your tomatoes |
Rain Catch by Roof Size
Gallons collected from 1 inch of rain at 85 percent roof runoff.
| Roof Area | Gallons per Inch of Rain | 55-gallon Barrels Filled |
|---|---|---|
| 100 sq ft | about 53 | about 1 |
| 250 sq ft | about 132 | about 2.4 |
| 500 sq ft | about 265 | about 4.8 |
| 1,000 sq ft | about 530 | about 9.6 |
How Is the Rain Catch Worked Out?
One inch of water over one square foot is 144 cubic inches, and a gallon is 231 cubic inches, so each square foot yields 0.623 gallons per inch of rain. Roofs lose some to splash and evaporation, so the tool keeps 85 percent.
Stored = the smaller of rain collected and total barrel capacity
Tomato water per week = plants x weekly need per plant
Days of watering = stored gallons / weekly need x 7
Weekly need per plant matches our watering calculator: 1 to 2 inches of water a week over about 4 square feet per in-ground plant, a little more for raised beds, and 3.5 to 10.5 gallons for a container plant.
Worked Example
Say 250 square feet of shed roof drains to one 55-gallon barrel, and it rains 1 inch. The roof sheds about 132 gallons, so the barrel fills and about 77 gallons overflow. It takes only about 0.4 inches of rain to fill it. Six in-ground tomatoes in warm weather use about 22 gallons a week, so the full barrel lasts about 17 days.
Factors That Change the Catch
- Roof area: only the section draining to your downspout counts, not the whole roof.
- Roof material: smooth metal sheds more than rough shingles or gravel.
- Gutter condition: clogged or leaking gutters lose water before it reaches the barrel.
- First flush: diverting the first dirty runoff lowers the catch a little but cleans it.
- Storm size: in a big storm a single barrel fills fast and the rest overflows.
Understanding Your Outputs
| Output | Meaning | Best Used For |
|---|---|---|
| Rain collected | Roof runoff after losses | Sizing your storage |
| Overflow | Water your barrels cannot hold | Deciding on extra barrels |
| Days of watering | How long the stored water lasts | Planning dry spells |
When to Use It
Use it when choosing how many barrels to buy, or to see how long your stored water carries tomatoes through a dry spell. Pair it with the watering calculator for weekly needs and the rainfall watering calculator to see how much top-up rain already covers.
Common Mistakes to Avoid
- Counting the whole roof: use only the area that drains to the barrel’s downspout.
- Too little storage: one barrel often overflows in a single inch of rain.
- Leaving it uncovered: screen the inlet to keep out mosquitoes and debris.
- Watering the fruit: roof water is not drinking water; water the soil at the base of plants.
- No overflow route: send overflow away from your foundation.
Accuracy and Limitations
This is a planning estimate based on standard rainwater math.
What It Calculates: rain collected, storage filled, and days of tomato watering.
What It Does Not Know: your exact roof surface, gutter losses, or evaporation from the barrel. Real catch can be a little lower or higher than the 85 percent used here.
Methodology
Method: rain collected is roof area times rainfall times 0.623 gallons per square foot per inch, times a runoff coefficient of 0.85. Why 0.623: one inch over one square foot is 144 cubic inches, and a US gallon is 231 cubic inches. Runoff: rooftops typically shed 75 to 95 percent of rain; 0.85 is the common middle value. Storage: the smaller of rain collected and total barrel capacity; the rest is overflow. Tomato use: the same weekly figures as our watering calculator, 1, 1.5, or 2 inches a week over about 4 square feet per in-ground plant (15 percent more for raised beds), or 3.5, 7, or 10.5 gallons per container plant. Rounding: gallons to whole numbers, inches to one decimal.
Sources: University of Arizona Water Wise – Harvest Rain, Take Care of Texas – Calculating Your Rainwater Harvest, The Old Farmer’s Almanac – Tomatoes. Last reviewed: September 26, 2026 · Reviewed by Aiza Anwar.
Frequently Asked Questions
How much water does 1 inch of rain put in a rain barrel?
About 0.53 gallons per square foot of roof after typical losses. A 250 square foot roof section sheds roughly 132 gallons, enough to fill two 55-gallon barrels.
How much rain does it take to fill a 55-gallon barrel?
It depends on the roof area. From 250 square feet it takes only about 0.4 inches of rain; from 100 square feet, about 1 inch.
Is rain barrel water safe for tomatoes?
Yes, for watering the soil. Roof runoff is not drinking water, so apply it at the base of the plants rather than over the fruit, and keep the barrel covered.
How long will a rain barrel water my tomatoes?
A full 55-gallon barrel waters six in-ground tomatoes for about two and a half weeks in warm weather, less in hot weather or in containers.
How many rain barrels do I need?
Enough to hold a typical storm from your roof section. If one inch of rain overflows your barrel, linking a second or third barrel captures more.
What is the 0.623 in rainwater math?
It converts inches of rain on square feet of roof into gallons. One inch over one square foot is 144 cubic inches, and 144 divided by 231 cubic inches per gallon is 0.623.
How do I measure my roof area?
Measure the footprint of the roof section that drains to your downspout, its length times its width on the ground. The roof slope does not add catchment area.
Should I divert the first flush of rain?
It helps. The first runoff carries dust and debris from the roof, so a simple first-flush diverter keeps the barrel water cleaner.
How do I stop mosquitoes in a rain barrel?
Keep the barrel sealed and screen the inlet with fine mesh so mosquitoes cannot lay eggs, and use the water regularly.
Related Tomato Tools
Work It Out with the Watering Calculator
This calculator gives a planning estimate using standard rainwater math. Real catch depends on your roof, gutters, and weather. Roof runoff is not drinking water; use it on the soil around plants. Check any local rules on rainwater collection. Spotted an error? Tell us here.