Testing Out Stair Geometry Calculator - Need Help Testing

[jar546]

I need some help testing this and giving feedback. This one has a metric or standard option.

Stair Geometry Calculator - 2024 IRC

Interactive stair tool using IRC 2024 Section R318.7. Calculate risers, treads, and check code compliance.

www.thebuildingcodeforum.com

 

[Yankee Chronicler]

Doesn't work for me. This state has amended the IRC to allow "old-fashioned" 8-1/4" x 9" stairs.-- which were the standard for decades. I input a total height of 109 inches and a tread depth of 9 inched. Result:

Please enter valid input values.

 

[jar546]

Doesn't work for me. This state has amended the IRC to allow "old-fashioned" 8-1/4" x 9" stairs.-- which were the standard for decades. I input a total height of 109 inches and a tread depth of 9 inched. Result:

Try it again. I adjusted the code to add rise and run that were not IRC specific for an override.

 

[BN4537]

Try it again. I adjusted the code to add rise and run that were not IRC specific for an override.

The manual input function you added works on my end, also lets you use it for IBC stairs now since they are 7" risers. I also tried it with 12" risers, that works too lol.

 

[jar546]

The manual input function you added works on my end, also lets you use it for IBC stairs now since they are 7" risers. I also tried it with 12" risers, that works too lol.

Manual input overrides it all and just does the math.

 

[SCBO1]

JAR, while your at it, should do a concrete calculator for slabs and piers.

Where do you find all the time to do all of this?
Are you related to Elon?

 

[jar546]

JAR, while your at it, should do a concrete calculator for slabs and piers.

Bluebeam does that.

 

[ICE]

Limitless fun.

 

[jar546]

Changes made, new version available.

 

[Yankee Chronicler]

New run for a hypothetical residential stair with a rise of 109 inches.



So the calculator works -- but it doesn't optimize the stair based on an ideal tread-riser relationship. A proportional stair with 7.75" risers should have 9.5" treads, not 9".

It's probably asking too much for a simplified stair calculator to take that into account. Which basically means this calculator can be used as a code check, but should not be used as a design tool.

 

[jar546]

It's probably asking too much for a simplified stair calculator to take that into account. Which basically means this calculator can be used as a code check, but should not be used as a design tool.

Agree, but I will try to make it both.

 

[jar546]

New run for a hypothetical residential stair with a rise of 109 inches.

View attachment 15819

So the calculator works -- but it doesn't optimize the stair based on an ideal tread-riser relationship. A proportional stair with 7.75" risers should have 9.5" treads, not 9".

It's probably asking too much for a simplified stair calculator to take that into account. Which basically means this calculator can be used as a code check, but should not be used as a design tool.

I am thinking about putting in a maximum run length to see of you can even put stairs in place and be compliant.

Now that I read your feedback again, I am a bit confused as to what you are trying to do and why. The calculator isn’t meant to optimize stair comfort; it’s just checking code compliance per IRC 2024. The 3/8" variation is valid during layout because the math ensures equal risers if you divide the total rise cleanly. That check is there to catch bad inputs, not to second-guess a proper layout. Could I add proportional design logic? Sure, but that’s not a code requirement, and it’s outside the scope of what this tool is for.

 

[jar546]

I did just make one small tweak. The fractions were not correct. Instead of 7 1/4", it was showing 7 4/16". That is fixed and yet another reason to go metric which is why it is an option.

 

[Yankee Chronicler]

I am thinking about putting in a maximum run length to see of you can even put stairs in place and be compliant.

Now that I read your feedback again, I am a bit confused as to what you are trying to do and why. The calculator isn’t meant to optimize stair comfort; it’s just checking code compliance per IRC 2024. The 3/8" variation is valid during layout because the math ensures equal risers if you divide the total rise cleanly. That check is there to catch bad inputs, not to second-guess a proper layout. Could I add proportional design logic? Sure, but that’s not a code requirement, and it’s outside the scope of what this tool is for.

This is why I said using it as a design tool may be more than what's possible for such an on-line tool. The problem is that when you enter a tread dimension, your checker maintains that tread dimension. But as the riser height decreases, the tread depth should increase. The old Architectural Graphic Standards had a page with a graphic chart that made this easy, but that's packed away in a box somewhere in my attic so I can't post it.

The basic rule is that 2R + T should equal between 24 and 25. Thus, for a hypothetical code conforming 7:11 stair, you would get

(2x7) + 11 = 14 + 11 = 25

But the riser height rarely works out to exactly 7 inches so, as the riser height drops, the tread depth should increase. If the riser is 6.75" we should see

(2x6.75) + T = 24<>25
13.5 + T = 24<>25
T = 10.5 <> 11.5

If the riser drops to 6.5,

(2x6.5) + T = 24<>25
13 + T = 24<>25
T = 11<>12

When I was doing working drawings, there was another formula that we used as a check, but I don't recall that one. It's probably in Graphic Standards. Mostly, though, once we knew the riser height we used the graphic chart in Graphic Standards to select the tread depth for an optimal rise:run ratio.

Here it is -- two versions. The one on the left is from a very old edition, as evidenced by the hand-drawn lettering. The one on the right looks like the one I remember, from an edition of around 1970. Note that this provides several formulas for fine-tuning the tread:riser ratio.