SettingsSettings for this calculationUS
The full, un-notched depth of the beam.
The beam's depth before the notch — the section it would have if nobody had cut it. The notch's effect is judged as a proportion of this, and it is severe: shear capacity falls with roughly the square of the remaining depth ratio, so a notch taking a quarter of the depth removes far more than a quarter of the shear strength. A notch on the tension face at a support is the worst case and the most common one.
How deep the end notch is cut into the beam, measured from the notched face.
NDS 3.4.3 applies to a notch on the tension side at the end of the member — a different, more conservative check applies to notches at other locations along the span.
Shear stress amplification factor
1.452 ×
- Net depth after notch
- 9.75 in
They open the calculator with your figures already in it
Timber Beam Notch Reduction Factor Calculator: 1.45 × — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- NDS (National Design Specification) 3.4.3: for a beam notched on the tension side at the end of the member, actual shear stress is amplified relative to net depth by the factor (full depth ÷ net depth)²
Inputs used
- Full Beam Depth
- 11.75 in
- Notch Depth
- 2 in
Intermediate steps
- Net depth after notch
- 9.75 in
What this calculation does not cover
- This returns the amplification factor on its own. It never asks for the shear force at the support, the beam breadth or the species allowable shear value, so it cannot say whether the notched beam is adequate — a 1.44 factor is harmless on a lightly loaded joist and decisive on one already sitting close to its allowable shear stress.
- The factor multiplies the shear stress worked out on the net depth below the notch — 3V ÷ 2b·dn — not on the full section. Apply it to a gross-section stress of 3V ÷ 2bd instead and the answer falls short by a further full-depth to net-depth ratio: for the 300 mm (12 in) beam with a 50 mm (2 in) notch used as the example, the correct stress is 20 per cent higher than that mistake gives.
- Nothing here enforces how deep a notch is allowed to be. The inputs will accept a 150 mm (6 in) notch in a 300 mm (12 in) joist and return 4.00 without objection, whereas end notches in sawn bending members are normally capped near a quarter of the depth, with a shallower limit away from the support and no notching through the middle third of the span; the exact fractions depend on the code and edition you are working to.
- The squared ratio assumes a square notch with a sharp re-entrant corner and gives no credit for tapering or bevelling the cut, which is the usual way of easing that stress concentration. It also frames the problem as a shear stress, when the failure that governs is a split running along the grain from the corner in tension perpendicular to the grain — a seasoning check already present at that corner can start it below the calculated stress.
- The provision behind this factor is written for solid rectangular sawn timber. Glulam, LVL and I-joists are governed by separate rules — I-joist flanges must not be notched at all, and tension-face notches in glulam have their own provisions — yet the calculator will still return a number if you type those dimensions in.
- An end notch also moves the bearing onto the underside of the cut, and compression perpendicular to the grain over that shortened seat is not checked here at all. A notch that passes the shear amplification check can still crush at the support.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- NDS (National Design Specification) 3.4.3: for a beam notched on the tension side at the end of the member, actual shear stress is amplified relative to net depth by the factor (full depth ÷ net depth)²
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