Understanding your result
The headline is the number of pipe pieces, with the elbows and splash blocks in the subtitle. The stats give the elbows, extensions, splash blocks and straps; the breakdown lists each part and the discharge.
The number of downspouts comes from the downspout calculator; the eave height from the ridge height calculator’s fascia row or a tape; and if a barrel sits on the downspout, its overflow from the rain barrel calculator.
How we calculate this
The vertical pipe is counted across all downspouts, since one 10-foot piece can be cut for two short runs. Buried pipe adds 2 feet per downspout for the bend and the adapter at the base.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Discharge distance | 5 ft | Common guidance: 4 to 6 ft, 10 ft on expansive soils |
| Pipe length | 10 ft | Standard downspout pipe |
| Elbows | 3 per downspout | Typical installation |
| Straps | every 6 ft, at least 2 | Installer practice |
| Grade away from the house | 6 in in 10 ft | IRC R401.3 |
Assumptions last reviewed October 7, 2026.
The calculator does not size dry wells, does not design drainage across a property, and assumes each downspout runs the same height. The downspout placement guide covers where they should go.
Two worked examples
Four downspouts with surface extensions
Four downspouts, 10-foot eave, 5-foot discharge, rigid pipe.
- Vertical: 40 ft, 4 pieces; extensions: 4
- Pipe pieces: 8; elbows: 12; straps: 8; splash blocks: 4
Six downspouts buried to daylight
Six downspouts, 18-foot eave, 10-foot discharge, buried drain.
- Vertical: 108 ft, 11 pieces; elbows: 18; straps: 18
- Buried pipe: 6 × 12 = 72 ft with adapters and pop-up emitters
Where to find your inputs
Downspouts. From the downspout calculator or the house.
Eave height. Ground to the gutter.
Distance. 4 to 6 feet minimum, more on clay soils.
Common mistakes
- Discharging at the wall. Water returns to the footings.
- Splash block only. About 2 feet is not enough on flat ground.
- Into the foundation drain. Never.
- Buried pipe without slope. It fills and freezes.
- Missing elbows. Three per downspout.
- Extensions across a path. Route them or bury them.
Questions people ask
- How far should downspouts discharge from the house?
- At least 4 to 6 feet from the foundation on most soils, and 10 feet on expansive clay or beside a basement. The ground should also fall away from the house, about 6 inches over the first 10 feet (IRC R401.3), so water keeps moving.
- Are splash blocks enough?
- A splash block spreads water about 2 feet from the wall, short of the 4 to 6 feet most foundations need. Where the ground falls away steeply it may do; elsewhere add an extension.
- How many elbows does a downspout need?
- Three: two at the top to bring the pipe from the gutter outlet back to the wall (an A and a B elbow, depending on direction), and one at the bottom to turn the water away. Four downspouts take twelve.
- How many downspout straps?
- One every 6 feet or so, at least two per downspout: a 10-foot downspout takes two, an 18-foot one three or four.
- Should downspouts be buried?
- Buried solid pipe to daylight or a dry well keeps the yard clear and carries water further, but needs a slope, a cleanout and a pop-up emitter at the end, and it can freeze or clog. Never connect downspouts to a foundation drain.
- Can a rain barrel replace an extension?
- No, the barrel overflows in most storms, so the overflow still needs an extension. The rain barrel calculator shows how quickly it fills.