Rain Barrel Calculator

This rain barrel calculator shows how much water a section of your roof can send to a rain barrel, and how many barrels a storm would fill. Enter the size of the roof area that drains to your downspout, the rainfall, how efficiently your roof and gutters collect it, and the size of your barrel. The calculator returns the gallons collected, the gallons per inch of rain, the number of barrels that storm would fill, and how little rain it takes to fill a single barrel. Most people are surprised by that last figure.

How to use the rain barrel calculator

1. Measure the roof section that drains to the downspout. Use the footprint of the roof as seen from above, measured on the ground, not the sloped length of the roof surface. A steep roof and a flat roof over the same footprint catch the same amount of rain.
2. Enter the rainfall. Use a single storm to see how quickly a barrel fills, or a monthly average to plan storage.
3. Set the collection efficiency. Some rain splashes off, evaporates or overshoots the gutters. Most roofs collect about 75 to 90 percent.
4. Enter your barrel size. The standard rain barrel holds 55 gallons, but sizes from about 35 to over 100 gallons are common.

How much water does a roof collect?

The math is simple. One inch of rain falling on one square foot equals about 0.62 gallons. Multiply by the roof area and the rainfall, then by the collection efficiency.

Gallons = roof area (sq ft) × rainfall (in) × 0.62 × efficiency

A useful rule of thumb: one inch of rain on 1,000 square feet of roof yields about 623 gallons before losses, or roughly 500 to 560 gallons in practice.

Worked example: one downspout

A roof section 20 feet by 15 feet, 300 square feet, drains to one downspout. One inch of rain at 80 percent efficiency produces 300 × 1 × 0.62 × 0.8 ≈ 150 gallons. That fills almost three 55-gallon barrels. It takes only about 0.37 inches of rain to fill one barrel, which means a single barrel will overflow in most moderate storms.

Worked example: a year of rain

A 1,000-square-foot roof in a region that gets about 40 inches of rain a year can collect roughly 623 × 40 × 0.8 ≈ 19,900 gallons over the year. Rain barrels can only store a small part of that, which is why storage, not catchment, is usually the limit.

Gallons per storm at a glance

Figures assume 80 percent collection efficiency.

How many rain barrels do I need?

Because roofs collect so much water, almost any downspout can fill a barrel quickly. The real question is how much water you want to store and use between rains. Start from your garden’s needs.

• Weekly use: a 100-square-foot vegetable garden needs about 62 gallons a week in typical summer weather. The garden watering calculator works out your figure.
• Dry spells: decide how many weeks of dry weather you want to cover. Two weeks for that garden is about 125 gallons, or about two and a half 55-gallon barrels.
• Space and budget: each barrel needs a level, sturdy base. A full 55-gallon barrel weighs over 450 pounds.

Many gardeners link two or three barrels together, so overflow from the first fills the next. Larger tanks of several hundred gallons are an option where space allows, and they often cost less per gallon of storage than several small barrels.

Measuring your roof catchment

Walk around the house and note which downspout drains which section of roof. For a simple gable roof with a downspout at each corner, each downspout usually drains about a quarter of the roof, though gutter slope can change this. Measure the length and width of the building footprint covered by that roof section, including the overhang.

Roof slope does not matter for catchment, because rain falls vertically. A 30-by-40-foot house has a 1,200-square-foot footprint whether the roof is nearly flat or steeply pitched. If you can only measure the house, add the width of the eaves on each side.

Garage, shed and greenhouse roofs are often ideal places for a barrel near the garden, even though they are smaller. A 10-by-12-foot shed roof collects about 60 gallons per inch of rain, enough to fill a barrel in a single good storm.

Setting up a rain barrel

1. Raise the barrel. Set it on a sturdy, level stand of concrete blocks or a purpose-built platform. Height increases water pressure and lets you fit a watering can under the spigot. Each foot of height adds only about 0.43 psi, so gravity-fed barrels deliver a gentle flow.
2. Connect the downspout. Cut the downspout and add a diverter or a flexible elbow that directs water into the barrel’s screened inlet.
3. Add an overflow. Fit an overflow hose that carries excess water at least several feet away from the foundation, or into a second barrel or a rain garden.
4. Screen every opening. Fine mesh keeps out mosquitoes, leaves and debris. Standing water without screens can breed mosquitoes within about a week.
5. Consider a first-flush diverter. It sends the first, dirtiest water from each storm away from the barrel, so less roof debris ends up stored.

Using rainwater in the garden

Rainwater is naturally soft and free of the chlorine found in many municipal supplies, and plants respond well to it. A few precautions make sense when using roof runoff on food crops:

• Water the soil, not the leaves, especially on leafy greens and crops eaten raw.
• Avoid using collected water on edible plants just before harvest, and wash produce well.
• Do not drink it or use it for cooking without proper treatment.
• Consider your roof material. Many extension programs suggest extra caution with water from treated wood shakes or older roofs, and prefer metal or newer shingle roofs for harvesting.

Rain barrels pair especially well with soaker hoses. Most drip emitters are designed for higher pressure than a barrel provides, so check that any drip products you connect are rated for low-pressure, gravity-fed use. The drip irrigation calculator helps size beds and run times.

Is rainwater harvesting legal?

Collecting rain from your roof in barrels is legal in most US states, and many cities and utilities actively encourage it with rebates or discounted barrels. A few states regulate it. Colorado, for example, allows households to use up to two barrels with a combined capacity of 110 gallons. Some other states have registration rules or limits on larger systems. Check your state and local rules before installing large storage tanks, and ask your water utility about rebate programs.

Winter care

In regions where temperatures drop below freezing, water left in a barrel can freeze, expand and crack the barrel or its fittings. Before the first hard freeze, drain the barrel, disconnect it from the downspout, restore the downspout so water drains away from the house, and store the barrel upside down or indoors. In mild climates, barrels can stay in use year-round. Just keep screens clean and check the overflow after heavy storms.

Common mistakes

• No overflow. Barrels fill quickly. Without an overflow, water spills around the base and toward the foundation.
• Unscreened openings that let in mosquitoes and debris.
• Unstable bases. A full barrel weighs as much as several people. Use a solid, level base.
• Expecting drip-level pressure. A barrel’s gravity pressure is low, so choose soaker hoses or low-pressure drip parts.
• Forgetting winter. Freezing water can split barrels and fittings.

Choosing a rain barrel

Rain barrels range from simple recycled food-grade drums to decorative planters with built-in storage. Whatever the style, a good barrel has a few essential features:

• Food-grade, opaque plastic or metal. Opaque material blocks light and limits algae growth. Avoid barrels that previously held chemicals.
• A screened inlet to keep out mosquitoes, leaves and debris.
• A spigot near the bottom for a watering can or hose, and ideally a second outlet for linking barrels.
• An overflow port near the top that can take a hose to carry excess water away.
• A secure lid, important if children or pets are around.

Many cities, counties and water utilities run rain barrel programs that sell good barrels at a discount or offer rebates, so check before buying.

Overflow and rain gardens

Because a barrel fills so quickly, most of the water from a large storm will overflow. Plan where that water goes. Direct the overflow hose at least several feet from the foundation, onto a lawn, a gravel splash area or a planted bed that can absorb it.

A rain garden is an excellent destination for overflow. It is a shallow planted depression that collects runoff and lets it soak slowly into the ground, filtering it on the way. Rain gardens are usually planted with native perennials, grasses and shrubs that tolerate both wet spells and dry periods. They reduce runoff to storm drains, create habitat for pollinators, and turn a drainage problem into a garden feature. Locate a rain garden at least 10 feet from the house foundation, and avoid areas over septic systems or utility lines.

Using rainwater for drip and soaker hoses

A barrel raised on blocks can feed a soaker hose by gravity, which is one of the simplest ways to use stored rainwater. Connect the hose to the barrel spigot, open it, and the water seeps slowly into the bed. Because gravity pressure is low, keep soaker runs short and avoid lifting them above the barrel outlet. For a more even flow, choose drip products sold for gravity or low-pressure systems.

Maintaining your rain barrel

Rain barrels need little care, but a few checks keep them working well and the water clean.

• Clean the gutters in spring and fall, so leaves and grit do not wash into the barrel.
• Check the screens after storms and replace any that are torn.
• Use the water regularly. Water that sits for weeks is more likely to grow algae and breed mosquitoes if a screen fails.
• Rinse the barrel once a year. Drain it, scrub out sediment, and rinse well. A dilute vinegar solution helps remove buildup.
• Inspect fittings for leaks at the spigot and overflow.

Frequently asked questions