Downspout elbows & offsets calculator
How many elbows to buy? Count 2 at the top of each downspout plus 1 at the bottom, and 2 more for every soffit offset. Get the fitting list right before the parts run.
Calculator
With 3 downspouts and 2 soffit offsets you need about 13 elbows. Count 2 elbows at the top of each downspout to bring water back to the wall plus 1 at the bottom; add 2 more per soffit offset.
Runs to the store multiply when the elbow count is a guess. It isn’t a guess. Each downspout takes two elbows at the top to walk the outlet back under the eave to the wall, and one at the bottom to turn water away at grade. Every soffit offset — the jog around a deep overhang — adds a pair.
Set the per-downspout number lower only if your outlets drop straight down against the fascia with no reach-back. Otherwise leave it at three.
Formula
elbows = downspouts × elbows_per_downspout + offsets × 2
Default elbows_per_downspout = 3 (two top + one bottom); each soffit offset = one pair.
Worked example
3 downspouts, 2 soffit offsets, 3 elbows each:
3 × 3 = 9 from the downspouts, plus 2 × 2 = 4 from the offsets → 13 elbows.
Buy a couple of spares — a cut mistake or a tight corner eats fittings, and the 45°/75° angles aren’t always both in stock.
Getting the fitting list right
Top pair, bottom single. The two top elbows are the “offset” that brings the outlet back to the wall; the bottom elbow aims the discharge outward before the extension.
Deep soffits need a jog. A wide roof overhang can force an extra pair to reach the wall — that is the soffit-offset input.
Match elbow style to the downspout. Rectangular downspouts use A/B-style elbows (front and side throw); round downspouts use round elbows. Mixing them won’t seat cleanly.
This counts fittings only. For the pipe and its cost, use the downspout cost tool below.
Reference table
Count the fittings the same way every crew does it:
| Fitting | How many | Why |
|---|---|---|
| Top elbows | 2 per downspout | Two 45°/75° elbows kick the outlet back to the wall. |
| Bottom elbow | 1 per downspout | Turns water away from the foundation at grade. |
| Soffit offset | 2 elbows each | A pair to jog around a deep soffit overhang. |
Default is 3 elbows per downspout (two top + one bottom); add 2 more for every soffit offset. Adjust the per-downspout count if your outlets sit tight to the wall.