Tank size, first-flush diverter and cost from your roof's actual catchment.
Estimates are planning aids, not a professional quote. Double-check quantities and product coverage — and any local code or permit requirements — before you buy.
⚠ Each downspout would need about 13.8 ft of 4" standpipe to hold its share of the first-flush volume, which is taller than most single-storey downspouts can carry. Either capture more downspouts to split the load, or switch to a wider chamber — 6" pipe holds roughly 2.3× as much per foot as 4" — or a commercial first-flush device rated to the volume.
⚠ Roof-harvested rainwater is for irrigation and other non-potable uses only. Drinking it, or using it anywhere it could contact food, requires additional filtration and a certified disinfection system on top of everything priced here.
⚠ Rainwater harvesting is regulated or requires a permit in some states and municipalities (Colorado is the best-known example) — check your state and local rules before installing a system this size.
| Plumbing & Heating | $303–$409 |
| Drainage | $275–$373 |
| Soil & Fill | $13–$18 |
| Total | $591–$800 |
Excludes tools, delivery, tax and permit fees. Labor not included — this is a DIY materials estimate.
These are affiliate links: we may earn a commission if you buy through them, at no extra cost to you. It never changes what the calculator recommends — the shopping list comes from your dimensions, not from what pays.
About this calculator
The Rainwater Harvesting Calculator sizes a collection system from your roof and your climate instead of a guess at what tank looks about right. Enter the roof area that actually drains to your downspouts, your area's average annual rainfall, and how much water your garden beds draw in a typical week, and it works out how many gallons a year the roof can realistically deliver against how many tanks to buy to bridge a dry stretch.
It also sizes the parts gutter calculators stop short of — a first-flush diverter that keeps the first dirty slug of roof-wash out of the tank, a leaf screen at every downspout, overflow routing once the tank is full, and an optional transfer pump if the garden sits uphill from the tank. Pick a rain barrel, an IBC tote or a poly cistern and every line reprices instantly.
Step by step
A step-by-step walkthrough to go with your plan above. Follow the product instructions and any local code or permit requirements as you work.
Use your roof area, local rainfall and garden's weekly demand to get a target storage volume and confirm the roof can realistically refill it — a tank far bigger than what the roof or the garden can cycle just sits full.
Level a spot within reach of the downspout you're tying into and lay a compacted gravel bed under each tank's footprint so it can't settle unevenly once it's full and heavy.
Cut the downspout at the diverter's height, install the diverter body, and route its outlet toward the first-flush riser rather than straight to the tank.
Plumb the capped vertical pipe ahead of the tank inlet, sized to the target diverted volume, with a slow-drain valve at the bottom so it empties and resets between storms.
Position the tank on the pad, cut the bulkhead opening, and install the fitting with its gaskets snug on both sides of the tank wall for a watertight seal.
Run the overflow line from a high fitting near the top of the tank out to a safe daylight point well clear of the foundation, so a full tank sheds excess water instead of backing up at the inlet.
Install the spigot at the bulkhead for hose use, and if the garden needs more than gravity can deliver, plumb in the transfer pump sized to the lift.
Run water through the whole system to check every joint, confirm the overflow sheds cleanly, and screen every open inlet so standing water can't become a mosquito breeding site.
Equip the job
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Keep planning
Garden & Yard
Answers
One inch of rain on one square foot of roof yields about 0.623 gallons, so a 1,200 sq ft roof in a 38-inch-a-year climate has roughly 28,400 gallons of rain falling on it annually. You never capture all of it — roof material, splash-out, evaporation and the first-flush diversion all take a cut, which is why this calculator discounts the raw number by a runoff coefficient and an 85% capture efficiency before it ever reaches the tank sizing.
The first few gallons off any roof carry dust, pollen, bird droppings and shingle grit that built up since the last rain, and letting that straight into your tank fouls the water and clogs the outlet over time. A first-flush diverter is a length of capped pipe plumbed ahead of the tank inlet that fills with that dirty slug first, then lets clean water pass through once it's full — this calculator sizes that pipe from your roof area and roof material, since asphalt sheds more grit per storm than metal.
Size it to your garden's draw, not to how much rain theoretically falls — a tank far bigger than what your beds use in a few weeks just sits full and never cycles. This calculator multiplies your weekly garden demand by the number of dry days you want the system to bridge (the autonomy buffer) to get a target storage volume, then rounds that up to a whole number of whichever tank size you pick, whether that's rain barrels, IBC totes or a single larger cistern.
If the tank sits above the beds it's feeding — on a stand, a hillside, or just a taller foundation than the garden — a gravity drip line or soaker hose can work with no pump at all, though flow will be modest. If the beds are level with or above the tank outlet, or you want real hose pressure, you need a transfer pump; this calculator sizes a small submersible pump for lifts up to about 10 feet and steps up to a surface-mounted pump with a pressure switch above that.
Not as collected off an asphalt or metal roof through the system priced here — it's suitable for irrigation, washing and other non-potable uses only. Turning it into drinking water requires additional filtration (typically a multi-stage sediment and carbon filter) plus a certified disinfection step like UV treatment or chlorination, on top of everything this calculator lists. Treat any system without that added treatment as garden and outdoor-use water only.
In most of the U.S. it's unrestricted or actively encouraged, sometimes with rebates for the equipment. A handful of states and municipalities regulate collection volume or require a permit above a certain tank size — Colorado is the most commonly cited example, having only broadly legalized it in recent years with specific limits. Check your state and local codes before you buy tanks, especially if you're planning a cistern-sized system rather than a couple of barrels.