Dryer Vent Length Chart: Max Duct Run by Elbows 2026

Every elbow costs you run

The International Residential Code (IRC M1502.4.5.1) caps a dryer vent at 35 ft of developed length from the dryer to the termination — and every elbow counts against it. A 90-degree elbow costs 5 ft of run; a 45-degree elbow costs 2.5 ft. On the defaults — straight length 20.00 ft, 2 × 90-degree elbows, 0 × 45-degree elbows — the effective length is 30.00 ft, the remaining allowance is 5.00 ft and the verdict is PASS — within the limit.

Dryer vent length chart — maximum straight duct length (ft)

At the defaults — straight length 20.00 ft, 2 × 90-degree elbows, 0 × 45-degree elbows — the effective length is 30.00 ft, the remaining allowance is 5.00 ft and the verdict is PASS — within the limit. Check the identities: effective length = straight length + 5 ft × n90 + 2.5 ft × n45; remaining allowance = 35 ft − effective length; maximum straight length = 35 ft − 5 ft × n90 − 2.5 ft × n45; verdict PASS when effective length ≤ 35 ft, FAIL when over. Rounding convention: totals are computed from unrounded values, so rounded parts may differ by a cent from their rounded sum.

The worked example, line by line

Setup: straight length 20.00 ft, 2 × 90-degree elbows, 0 × 45-degree elbows.

Line 1 — the elbows. Two 90-degree elbows at 5 ft each cost 2 × 5 ft = 10.00 ft. There are no 45-degree elbows, so they cost 0.00 ft.

Line 2 — the effective length. 20 ft straight + 10.00 ft of elbows = 30.00 ft.

Line 3 — the remaining allowance. 35 ft − 30.00 ft = 5.00 ft.

Line 4 — the verdict. 30.00 ft is within the 35 ft limit, so PASS — within the limit.

Reverse: the straight length at the limit with 2 × 90-degree elbows is 25.00 ft; at 25.00 ft the effective length is 35.00 ft, remaining 0.00 ft — PASS; at 25.01 ft the effective length is 35.01 ft, over by 0.01 ft — FAIL. The 90-degree elbows at the limit for a 20 ft straight run are 3; at 3 elbows the effective length is 35.00 ft — PASS; at 4 elbows it is 40.00 ft, over by 5.00 ft — FAIL. The 45-degree elbows at the limit for a 25 ft straight run are 4; at 4 elbows the effective length is 35.00 ft — PASS; at 5 elbows it is 37.50 ft, over by 2.50 ft — FAIL. The straight length at the limit with 3 × 90-degree + 1 × 45-degree is 17.50 ft; at 17.50 ft the effective length is 35.00 ft, remaining 0.00 ft — PASS; at 17.51 ft the effective length is 35.01 ft, over by 0.01 ft — FAIL. The maximum straight length with 4 × 90-degree elbows is 15.00 ft — the table cell. For 12.5 ft straight with 2 × 90-degree + 1 × 45-degree the effective length is 25.00 ft, remaining 10.00 ft — PASS.

Every branch, checked

25 ft straight, 2 × 90-deg: Effective 35.00 ft, remaining 0.00 ft — PASS — within the limit. Exactly at the limit — the verdict is PASS, with no run left.

30 ft straight, 2 × 90-deg: Effective 40.00 ft, remaining -5.00 ft — FAIL — over the limit by 5.00 ft. Five feet too long — the run fails by 5.00 ft.

15 ft straight, no elbows: Effective 15.00 ft, remaining 20.00 ft — PASS — within the limit. A short, straight run — 20.00 ft of allowance left.

20 ft straight, 4 × 45-deg: Effective 30.00 ft, remaining 5.00 ft — PASS — within the limit. Four gentle elbows cost only 10 ft, so the run passes with 5.00 ft left.

20 ft straight, 4 × 90-deg: Effective 40.00 ft, remaining -5.00 ft — FAIL — over the limit by 5.00 ft. Four sharp elbows cost 20 ft — the run fails by 5.00 ft.

10 ft straight, 1 × 90-deg + 2 × 45-deg: Effective 20.00 ft, remaining 15.00 ft — PASS — within the limit. A mixed run — 5 ft plus 5 ft of elbows — passes with 15.00 ft left.

0 ft straight, no elbows: Effective 0.00 ft, remaining 35.00 ft — PASS — within the limit. The dryer sits right at the wall — the full 35.00 ft is available.

35 ft straight, no elbows: Effective 35.00 ft, remaining 0.00 ft — PASS — within the limit. Exactly at the limit with no elbows at all — PASS, with no run left.

What the defaults assume

The model assumes 4-inch rigid metal duct — the code duct — and the IRC developed-length rule: 35 ft maximum, minus 5 ft per 90-degree elbow and 2.5 ft per 45-degree elbow. It assumes the straight length is measured along the duct, and that the termination is the outside vent, not the dryer’s own outlet. It does not model the dryer manufacturer’s own instructions, which may allow a longer run — if the manual allows more, the manual governs. Flexible foil or plastic duct is not allowed by code at all, whatever its length. With a 0 ft straight run and the default elbows the effective length is 10.00 ft, remaining 25.00 ft, PASS — within the limit. At the maximum inputs — 100 ft straight, 10 × 90-degree, 10 × 45-degree — the effective length is 175.00 ft, over by 140.00 ft, FAIL — over the limit by 140.00 ft. Rounding convention: totals are computed from unrounded values, so rounded parts may differ by a cent from their rounded sum; the reverses are nearest-value solutions, and the exact unrounded figures (25.00 ft, 3, 4, 17.50 ft, 15.00 ft, 25.00 ft) are stated wherever the rounded figures appear.

Output · formula · meaning

Who uses this

• The homeowner running a 20 ft vent: they learn the effective length is 30.00 ft — two elbows make it a 30 ft run — and they check the 25 ft branch (Effective 35.00 ft, remaining 0.00 ft — PASS — within the limit) to see the absolute limit with those elbows.
• The contractor with four sharp elbows: they learn a 20 ft run with four 90-degree elbows is 40.00 ft — over the limit by 5.00 ft — and they check the four-45-degree branch (Effective 30.00 ft, remaining 5.00 ft — PASS — within the limit) to see that gentler elbows pass.
• The remodeler routing around a joist: they learn a 10 ft run with one 90-degree and two 45-degree elbows is 20.00 ft — and they check the 17.50 ft reverse to see the longest straight run with three 90s and a 45.
• The inspector checking a long run: they learn the maximum straight length with four 90-degree elbows is 15.00 ft — and they check the table to see every elbow combination at a glance.

Accuracy & limitations

The math is the IRC developed-length rule: effective length = straight length + 5 ft × n90 + 2.5 ft × n45; remaining allowance = 35 ft − effective length; verdict PASS when effective length ≤ 35 ft, FAIL when over. The 2-decimal exact is shown where the rounding matters: the straight length at the limit with 2 × 90-degree elbows is 25.00 ft (shown as 25.00 ft); at 25.00 ft the effective length is 35.00 ft, at 25.01 ft it is 35.01 ft. The straight length at the limit with 3 × 90-degree + 1 × 45-degree is 17.50 ft (shown as 17.50 ft); at 17.50 ft the effective length is 35.00 ft, at 17.51 ft it is 35.01 ft. What the model cannot do is measure your duct: elbows hidden in a wall count the same as visible ones, and a crushed or kinked duct restricts airflow far more than its length suggests — the length limit is a proxy for airflow, not a guarantee of it. Check your dryer’s manual: some manufacturers allow a longer run with rigid duct, and the manual governs. For the duct size itself, see the duct size chart; for the air conditioner the vent serves, see the air conditioner size chart; for the dryer itself, see the repair vs replace dryer calculator. This is a calculator, not a building-code opinion.