Getting your two starting numbers right
This calculator only needs two real-world measurements. Get those right and everything downstream — overhang, board size, code check — follows automatically.
Find your structural tread depth
This is the "T" (run) from your rise-and-run layout or stringer cut list — the horizontal distance the stair advances per step, measured nosing-to-nosing. It is not the physical board size. Pull it from the Stair Rise and Run Calculator if you haven't already worked it out.
Pick your tread board
Measure or look up the actual front-to-back depth of the board, slab, or stone you're installing — a stock nominal "1x12" finishes at 11.25", a 2x12 at 11.25" as well, and many stone/composite treads run 11.5"–12". This is different from the code tread depth.
Choose what you're solving for
Know your board and want the resulting overhang? Use "Nosing Overhang" mode. Know the overhang you want and need to size a board to order? Switch to "Board Depth" mode instead.
Read the compliance chips
Green means the overhang, tread depth, and edge profile sit inside your selected code's limits. Red means the board is too deep, too shallow, or the edge is over-radiused — adjust before you cut.
The four formulas behind every result
Nothing on this page is a black box. Here's exactly what runs when you change a number.
Nosing Overhang
The physical board overhangs the riser below it by whatever depth is left over once the structural run is accounted for.
Board Depth Needed
The reverse calculation — the exact board size to order or cut to hit a chosen overhang.
Stair Angle
Uses the code tread depth (the run), never the physical board — the overhang doesn't change the walking angle.
2R + T Comfort Check
The same comfort formula used across this site's geometry tools, run against your tread depth and riser height together.
Sizing a stock 1x12 tread against a 10" run
A common real situation: your stringers are already cut to a 10" run, and you're deciding whether off-the-shelf tread stock will still pass code.
Start with the structural run
Your rise-and-run layout gives a 10" tread depth (code-measured, nosing-to-nosing) and a 7.5" riser — both already IRC-compliant on their own.
Check the stock board
A nominal "1x12" board finishes at 11.25" deep. Overhang = 11.25 − 10 = 1.25" — landing exactly on the IRC maximum of 1-1/4". It passes, but with zero margin.
Where it breaks
Some mills cut "1x12" closer to 11.5" actual, which is a common enough variance to matter. At 11.5" board depth, overhang becomes 11.5 − 10 = 1.5" — now over the 1.25" max and technically an inspection fail, even though nothing about your stringer layout changed.
The fix
Either rip the board down to 11.25" or less before installing, or open the structural run slightly (10.25" instead of 10") so a full-width 11.5" board still lands inside the 1.25" cap. This is exactly why board stock should be measured, not assumed, before it's cut to length.
This is the interaction this calculator is built to catch: a perfectly legal tread run can still fail inspection once a specific piece of lumber is put on it, and the two checks — run and board size — have to be run together, not separately.
Nosing rules by code, side by side
The three codes most projects run into. Figures marked unconfirmed should be checked against your local adopted code before you build.
| Code | Min Overhang | Max Overhang | Max Radius | Max Bevel | Nosing Required When |
|---|---|---|---|---|---|
| IRC 2021 (Residential) | 3/4" | 1-1/4" | 9/16" | 1/2" | Tread depth < 11", solid risers |
| IBC 2021 (Commercial) | not set VERIFY | 1-1/4" | 9/16" VERIFY | 1/2" VERIFY | Rare — IBC min tread is already 11" |
| ADA / ANSI A117.1 | not set VERIFY | 1-1/2" | 1/2" | n/a VERIFY | Applies on accessible routes VERIFY |
Reference only — always confirm against your local jurisdiction's adopted code edition and amendments before building.
Open risers, edge profiles & the 11" exception
Overhang length is only one part of the nosing rule. These three items trip up otherwise-correct stairs just as often.
The 11" exception
Once your structural run hits 11", IRC and IBC both drop the nosing overhang requirement entirely — the tread is judged deep enough on its own. This is why many modern flush-front stair designs use an 11"+ run instead of adding a lip.
Solid vs. open risers
Code text ties the nosing projection requirement to stairways "with solid risers." Open-riser stairs have no riser face for the tread to overhang, so the same limits may not apply the same way — confirm the exact wording your jurisdiction has adopted before assuming either way.
Radius vs. bevel
The front top edge of the nosing can be rounded over (radius) or angle-cut (bevel), but not both stacked past these limits. ADA is stricter on radius, capping it at 1/2" regardless of code edition.
Flight-to-flight uniformity
Same rule that governs riser and tread uniformity applies to nosing projection — the largest and smallest overhang in one flight can't differ by more than 3/8", even if every individual overhang is within the min/max range.
Common mistakes when sizing nosing
Most nosing-related inspection fails trace back to one of these five, not to bad carpentry.
Confusing board depth with tread depth
A deep board doesn't automatically mean a deep code tread — the code number is measured nosing-to-nosing and is set by your structural run, not by the lumber you buy.
Overhanging too far
Past 1-1/4" (IRC/IBC) the lip becomes a toe-catch hazard on the way up, not a safety feature — more overhang is not automatically safer.
Letting overhang drift between steps
A 3/4" overhang on step one and a 1-1/8" overhang on step six both pass the min/max range individually but can still fail the 3/8" flight-uniformity rule.
Assuming nominal lumber sizing
"1x12" isn't always exactly 11.25" — mill variance is common enough to push an at-the-limit overhang over the max, as in the worked example above.
Over-rounding the front edge
A generous bullnose profile can quietly exceed the 9/16" radius or 1/2" bevel cap, especially on stone or engineered stone treads cut with a router by feel rather than to a spec.
When to use a stair nosing calculator
Nosing sizing shows up any time a specific board meets a specific structural layout.
New Construction
Confirming a chosen tread stock still fits the stringer layout before ordering material.
Remodel with Existing Stringers
Working backward from a fixed structural run to find the maximum board depth allowed.
Retrofitting a Nose Trim Strip
Adding a separate nosing strip to a flush existing tread and checking the resulting projection.
Commercial / IBC Projects
Verifying nosing dimensions against IBC and, on accessible routes, ADA limits at once.
Lumber & Stone Selection
Comparing several stock tread sizes against one layout before placing a material order.
Pre-Inspection Checks
Catching an over- or under-sized overhang before a plan check flags it.