Floor Joist Calculator

Created by Kenneth Alambra
Reviewed by Rijk de Wet
Based on research by
American Wood Council Design Values for Joists and Rafters (2021)
Last updated: Jul 01, 2022

This floor joist calculator will help you when you buy floor joists for your next flooring or deck framing project. Using this tool, you can quickly find out how many floor joists you will need for any size of floor and joist spacing. You can also estimate the material cost you'll need to set aside, since this tool also works as a floor joist cost calculator.

This tool is also a floor joist span calculator (which you can use separately) to determine the maximum allowable span a floor joist can have.

Keep on reading to start learning:

  • What a floor joist is;
  • The typical floor joist spacings;
  • How to use this floor joist calculator;
  • How to calculate how many floor joists you need; and
  • How to calculate floor joist span.

💡 The floor joist calculator is meant for estimation purposes and not to substitute professional expertise.

What is a floor joist?

A floor joist is a part of a floor system that acts as a beam, supporting the load acting upon the attached flooring material. In a floor system, we use several floor joists properly spaced from each other, so that subflooring materials like plywood or wooden planks are safe to walk on.

Spacing floor joists too far from each other means we would need lesser floor joists to cover our entire flooring. However, the floor might feel a little bouncy due to the lack of support underneath it. On the other hand, we can install the floor joists closer to each other so more of them can support the flooring material. But that could mean a more costly floor system.

We typically space floor joists 16 inches apart (measured from the centers of the joists). That way, we can lay an 8-foot (or 96 inches) long subfloor across 7 joists, as shown in the illustration below:

Illustration of an 8-feet subfloor material on 7 floor joists spaced 16 inches on-center.

We can space our joists closer or farther than 16 inches, but the other typical on-center spacings are as follows:

Spacing (inches)

Number of joists under an 8-foot long subfloor material

12.0

9

16.0

7

19.2

6

24.0

5

Illustration of the typical floor joist on-center spacings.
The typical on-center spacing of floor joists for an 8-feet subfloor.

But what if your subfloor material's length is not a multiple of 8 feet? In that case, you can use this formula to determine the joist spacing that will suit your preferred number of joists to support your subfloor material:

s=Lsub(nsub1)\small s = \frac{L_\text{sub}}{(n_\text{sub} - 1)}

where:

  • ss is the joist on-center spacing;
  • LsubL_\text{sub} is the length of subfloor; and
  • nsubn_\text{sub} is the number of joists you want to support your subfloor material.

In this calculator, it is worth noting that we are only considering natural solid lumber joists. That means we've excluded the use of I-joists and open-web truss joists (these are engineered joists that span longer than solid lumber joists). Nevertheless, we still have a huge selection of wood species and lumber grades to choose from in this calculator. The data we've used for this calculator came from the 2021 Design Values For Joists And Rafters by the American Wood Council.

How to use this floor joist calculator

The first step in using this floor joist calculator is to choose whether you want to calculate floor joist count or floor joist span (the maximum length a particular floor joist size can span to support a specified load).

To use this tool as a floor joist cost calculator:

  1. Enter the length and width of your floor.
  2. Select the nominal joist size you wish to use. You can also enter the thickness and height of your floor joists using our calculator in Advanced mode (which you can activate at the bottom).
  3. Type in your preferred on-center joist spacing. It's advisable to choose a joist spacing among the typical values provided in the previous section of this text. Our floor joist calculator will then display the number and recommended length of floor joists and end joists you will need for your floor size. The end joists are the joists used to cover the ends of the floor joists. They run perpendicular to the floor joists and provide stability against the lateral movement of the floor joists.
  4. Enter the price per floor joist and end joist and wastage percentage to determine the approximate cost of your floor joists.

On the other hand, here are the steps you can follow when using this tool as a floor joist span calculator:

  1. Select the nominal joist size you want to analyze. Like when using this tool as a floor joist cost calculator, you can also see the actual thickness and height of your selected joist size in the Advanced mode of our calculator. You can also change these values for custom-sized joists. We also use these measurements to calculate the area moment of inertia.
  2. Enter the on-center joist spacing of the floor joists.
  3. Choose the wood species and its lumber grade. In this step, our tool will instantly display the modulus of elasticity of your selected wood.
  4. Then, pick the deflection limit you want to allow on this floor joist.
  5. Finally, enter the total load per unit area your floor will have to support. After completing these steps, our tool will then provide you with the maximum allowable span of your specified floor joist.

Now that we know how to use this calculator, how about we discuss how to determine the number of floor joists we need for a flooring project? Keep on reading!

How many floor joists do I need?

For us to estimate how much it costs to install floor joists, it is essential to learn how to determine how many floor joists we need for a project. Here is the formula we can use for that:

n=1+Lws\small n = 1 + \frac{L - w}{s}

where:

  • nn is the number of joists needed;
  • LL is the length of floor;
  • ww is the actual thickness of floor joist; and
  • ss is the on-center spacing of the floor joists.

If we get a value for nn with a decimal, we round up that value to the nearest whole number. That will be the number of floor joists we need. Take note that the length of floor is the measurement perpendicular to the floor joists.

If we can get lumber with a length equal to or greater than our floor's length, we can get two pieces of that lumber for our end joists.

Of course, you'll need to consider some budget for fasteners like nails and screws and other hardware like brackets and hangers. We did not cover these materials in our calculator, as their quantity and cost can vary greatly depending on the project and your preference.

How do I calculate floor joist span?

To find how far any particular size of floor joist can span, we need to combine two formulas for the vertical deflection, δ\delta. The first deflection formula we need determines the allowable sag of a floor joist using a ratio. According to the 2012 International Building Code, the sag or deflection of a floor joist subjected to dead and live loads should not exceed a measurement equal to the span of the floor joist, L (in inches), divided by 240240. In equation form, we express that as:

δ=L/240\small \delta = L / 240

We can also divide LL by 360360, 480480, 600600, or 720720, depending on what loading combination you wish to consider in your calculation or how much deflection you want to allow. As a rule of thumb, we use 240240 for a combination of dead and live loads our floor will have to carry.

We then simultaneously use that equation with this maximum allowable deflection formula based on the floor joist material composition, its cross-section, and the load it supports:

δ=5×u×L4384×E×I\small \delta = \frac{5\times u\times L^4}{384\times E\times I}

where:

  • δ\delta is the maximum allowable deflection of floor joists;
  • uu is the uniformly distributed load a floor joist will carry;
  • LL is the span of the floor joist;
  • EE is the modulus of elasticity; and
  • II is the area moment of inertia.

If you know the load that your floor will carry (say PP in terms of force per unit area), you have to multiply it by the on-center spacing (ss) that you plan for your joists to get its equivalent uniform distributed load along the joists:

u=P×s\small u = P\times s

For the modulus of elasticity, you can refer to this table from the American Wood Council:

Species

Modulus of Elasticity, E (×106 psi)

Select struct.

No. 1

No. 2

No.3

Stud

Const.

Standard

Utility

Alaska Cedar

1.4

1.3

1.2

1.1

1.1

1.2

1.1

1

Alaska Hemlock

1.7

1.6

1.5

1.4

1.4

1.4

1.3

1.2

Alaska Spruce

1.6

1.5

1.4

1.3

1.3

1.3

1.2

1.1

Alaska Yellow Cedar

1.5

1.4

1.3

1.2

1.2

1.3

1.1

1.1

Aspen

1.1

1.1

1

0.9

0.9

0.9

0.9

0.8

Baldcypress

1.4

1.4

1.3

1.2

1.2

1.2

1.1

1

Beech-Birch-Hickory

1.7

1.6

1.5

1.3

1.3

1.4

1.3

1.2

Coast Sitka Spruce

1.7

1.5

1.5

1.4

1.4

1.4

1.3

1.2

Cottonwood

1.2

1.2

1.1

1

1

1

0.9

0.9

Douglas Fir-Larch

1.9

1.7

1.6

1.4

1.4

1.5

1.4

1.3

Douglas Fir-Larch (North)

1.9

1.8

1.6

1.4

1.4

1.5

1.4

1.3

Douglas Fir-South

1.4

1.3

1.2

1.1

1.1

1.2

1.1

1

Eastern Hemlock-Balsam Fir

1.2

1.1

1.1

0.9

0.9

1

0.9

0.8

Eastern Hemlock-Tamarack

1.2

1.1

1.1

0.9

0.9

1

0.9

0.8

Eastern Softwoods

1.2

1.1

1.1

0.9

0.9

1

0.9

0.8

Eastern White Pine

1.2

1.1

1.1

0.9

0.9

1

0.9

0.8

Hem-Fir

1.6

1.5

1.5

1.3

1.2

1.3

1.2

1.1

Hem-Fir (North)

1.7

1.7

1.6

1.4

1.4

1.5

1.4

1.3

Mixed Maple

1.3

1.2

1.1

1

1

1.1

1

0.9

Mixed Oak

1.1

1

0.9

0.8

0.8

0.9

0.8

0.8

Mixed Southern Pine

1.6

1.5

1.4

1.2

1.2

1.3

1.2

1.1

Northern Red Oak

1.4

1.4

1.3

1.2

1.2

1.2

1.1

1

Northern Species

1.1

1.1

1.1

1

1

1

0.9

0.9

Northern White Cedar

0.8

0.7

0.7

0.6

0.6

0.7

0.6

0.6

Norway Spruce (North)

1.5

1.3

1.3

1.2

1.2

1.2

1.1

1.1

Red Maple

1.7

1.6

1.5

1.3

1.3

1.4

1.3

1.2

Red Oak

1.4

1.3

1.2

1.1

1.1

1.2

1.1

1

Redwood

1.4

1.3

1.2

1.1

0.9

0.9

0.9

0.8

Southern Pine

1.8

1.6

1.4

1.3

1.3

1.4

1.2

1.2

Spruce-Pine-Fir

1.5

1.4

1.4

1.2

1.2

1.3

1.2

1.1

Spruce-Pine-Fir (South)

1.3

1.2

1.1

1

1

1

0.9

0.9

Western Cedars

1.1

1

1

0.9

0.9

0.9

0.8

0.8

Western Woods

1.2

1.1

1

0.9

0.9

1

0.9

0.8

White Oak

1.1

1

0.9

0.8

0.8

0.9

0.8

0.8

Yellow Cedar

1.6

1.4

1.4

1.2

1.2

1.3

1.2

1.1

Yellow Poplar

1.5

1.4

1.3

1.2

1.2

1.3

1.1

1.1

We calculate the area moment of inertia using this formula:

I=w×h312\small I = \frac{w\times h^3}{12}

where:

  • II is the area moment of inertia;
  • ww is the actual width of floor joist; and
  • hh is the actual height of floor joist.

As you may have seen on our second deflection formula, the loading on our flooring is directly proportional to the deflection. That means that the larger the load is, the more deflection our floor joist experiences. On the other hand, the modulus of elasticity and the area moment of inertia are both indirectly proportional to the deflection. That means we would want lumber with a higher modulus of elasticity and a greater area moment of inertia.

Say we want to equate the second deflection formula to δ=L/240\delta = L / 240. We'll then use this equation to find the maximum allowable span:

5×u×L4384×E×I=L240L4L=384×E×I240×5×uL3=384×E×I1200×uL=384×E×I1200×u3L=8×E×I25×u3\small \begin{align*} \frac{5\times u\times L^4}{384\times E\times I}&=\frac{L}{240}\\\\ \frac{L^4}{L}&=\frac{384\times E\times I}{240\times 5\times u}\\\\ L^3&=\frac{384\times E\times I}{1200\times u}\\\\ L&=\sqrt[3]{\frac{384\times E\times I}{1200\times u}}\\\\ L&=\sqrt[3]{\frac{8\times E\times I}{25\times u}}\\\\ \end{align*}

Note that this is the most basic way to find how long a floor joist can span. You would have to determine the floor joist span on various load combinations and find the shortest among those values. For now, let this method be your starting point.

FAQ

How do I calculate the floor joist that I need?

To calculate how many floor joists you will need, let's say on a floor that is 10 feet (or 120 inches) long and using 1.5-inch thick floor joists at 16 inches on-center spacing:

  1. Subtract the width of your floor joist from your floor's length:
    120" − 1.5" = 118.5"
  2. Divide that difference by the sum of the on-center spacing of the floor joists:
    118.5" / 16" = 7.40625
  3. Add 1 to this value and round up the answer to the next whole number:
    7.40625 + 1 = 8.40625 ≈ 9 floor joists

What is the standard floor joist size?

The standard size of floor joists is 2" × 8". However, dimensions such as 2" × 10" and 2" × 12" are best suitable for longer spans. On the other hand, floor joist systems can also use 2" × 4" and 2" × 6" for small-sized floor areas.

How far can a 2x6 floor joist span without support?

A select structural grade Douglas Fir-Larch floor joist with a nominal size of 2" × 6" can span to about 12 feet long (supported only at both ends) if it is going to support 50 lbs per square foot of loading. Softer woods could span shorter given the same loading at the same nominal size.

How far can I span a 2x8 floor joist?

You can span a 2" × 8" floor joist as far as 12 feet using No. 2 Southern Pine lumber spaced at 16 inches on-center. For the same size floor joist and spacing, if you need to reach about 13 feet, you can use a stronger wood like No. 2 Douglas Fir-Larch.

What is the span of a 2x10 floor joist?

A 2" × 10" floor joist has a span of 20 feet at 12-inches on-center spacing. At 16 inches on-center spacing, the same-size floor joist can only span up to 18 feet. That only applies to floor joists with a wood strength of 1,900,000 pounds per square inch, like that of a select structural grade Douglas Fir-Larch.

Kenneth Alambra
Calculate...
floor joist cost
Floor details
Floor length
ft
Floor width
ft
Joist details
Nominal joist size
2" × 4"
On-center joist spacing
in
Materials needed
Floor joist length
ft
↳ Number of floor joists needed
floor joists
End joist length
ft
↳ Number of end joists needed
end joists
Floor joist cost
Price per floor joist
$
Price per end joist
$
Waste
%
Cost of floor joists
$
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