How many floor joists do I need?

Divide your floor's width by the on-center spacing (in the same unit), round up, and add 1 -- that gives your floor joist count before waste. For example, a 10 ft wide floor with joists 16 in (1.333 ft) on center needs ceiling(10 / 1.333) + 1 = 9 joists. Add a waste allowance (10% is typical) and 2 end joists, and you get the total pieces to buy. This calculator does that arithmetic for you automatically from your floor dimensions and spacing.

What is the standard spacing for floor joists?

16 inches on-center (OC) is the most common residential floor joist spacing in North America, though 12 in OC is used for heavier loads, and 19.2 in or 24 in OC are used with deeper joists or engineered I-joists rated for the wider spacing. Wider spacing means fewer joists (lower material cost) but requires a deeper or stronger joist to carry the same load without excess deflection.

How do I know if my floor joists are strong enough?

Whether a joist size safely spans a given distance depends on its species, grade, cross-section, on-center spacing, the load it carries and your deflection limit -- there is no single number that applies to every case. This calculator's Span mode computes a real answer for the US NDS 2018 method using your actual inputs, but it is not a substitute for checking your official local design standard's span tables or having a licensed structural engineer verify your specific floor before you build, especially for anything load-bearing or beyond a simple outbuilding.

Does this calculator use official AS 1720 span tables?

No -- while you can select AS 1720 as your project's design standard, this calculator's Span mode does not yet have AS 1720's span-table data implemented; it only computes real span results for the US NDS 2018 method today. For any Australian project, always check your final joist size against AS 1720 span tables or the span tables published by timber industry bodies for your species and grade, or engage a structural engineer.

Does this replace a building surveyor or structural engineer?

No. This is a planning and estimating tool for quantities and material cost, not a certified structural calculation. For anything load-bearing beyond a simple garden shed or deck, have your joist size, span and spacing confirmed by a structural engineer and checked against the National Construction Code before you build.

Floor Joist Calculator — AS 1720 (Australia)

Floor Joist Calculator: How Many Joists Do You Need?

Framing a floor comes down to two separate questions: how many joists do you need to buy for the footprint you're building, and is the joist size you have in mind actually strong enough for the span? This floor joist calculator answers both. In "Count" mode, enter your floor's length and width plus your chosen on-center spacing, and it works out the number of floor joists and end (rim) joists you need, with a waste allowance and a material cost estimate. In "Span" mode, enter a joist size, species/grade and load, and it calculates the maximum span that size can safely carry before deflecting more than your chosen limit.

This is a planning and estimating tool, not a substitute for a structural engineer or your local building code. Joist sizing is safety-critical: get it wrong and a floor can sag, bounce excessively, or in a worst case fail under load. Span results here are only fully computed under the US NDS 2018 method. Where a different design standard applies -- Eurocode 5 or AS 1720, for instance -- always check the maximum span against your official local span tables or a qualified engineer before you build, even if this calculator lets you select that standard.

How to Use the Floor Joist Calculator

Choose your calculation mode: "Count" to work out how many joists you need for a floor, or "Span" to check the maximum allowable span for a joist size.
In Span mode, select the design standard that applies to your project (US NDS 2018, Eurocode 5, or AS 1720).
Pick a nominal joist size (2x6 through 2x12) from the preset list, or enter a custom width and depth in inches.
In Span mode, also pick a species/grade preset (Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir, or Southern Pine) or enter a custom modulus of elasticity.
In Count mode, enter the floor length and width in feet, and your price per floor joist and per end joist.
In Span mode, enter the total load per square foot and choose a deflection limit -- L/360 is the standard default for residential floors, with stiffer options up to L/720 for tiled or sensitive floors.
Choose an on-center (OC) spacing preset -- 12, 16, 19.2 or 24 inches -- or enter a custom spacing.
In Count mode, adjust the waste allowance percentage if needed.
Read off the joist count and estimated cost (Count mode) or the maximum allowable span (Span mode).

The Math Behind the Calculation

Joist count uses the standard "fencepost" on-center spacing formula: count = ceiling(width / spacing) + 1. You add 1 because a spacing calculation counts the gaps between joists, not the joists themselves -- and there is always one more joist than there are gaps, with a joist sitting at each end. End (rim) joists are a fixed count of 2 -- one running along each long edge of the floor, framing the ends of the floor joists. A waste allowance (5-15%, 10% by default) is then applied to the floor joist count and rounded up, since you cannot buy a fraction of a joist.

Worked example: a 12 ft x 10 ft floor with joists spaced 16 in (1.333 ft) on center. Floor joist count = ceiling(10 / 1.333) + 1 = ceiling(7.5) + 1 = 8 + 1 = 9 joists, each 12 ft long (spanning the length). Adding a 10% waste allowance: 9 x 1.10 = 9.9, rounded up to 10 joists to buy. Add 2 end joists, each 10 ft long (running along the width) = 12 pieces of lumber in total for the floor frame.

Span mode works differently: rather than counting joists, it solves for the longest distance a single joist of a given size, species and grade can safely carry before it deflects (sags) more than your chosen limit under a given load. That depends on the joist's cross-section (width and depth), the species/grade's modulus of elasticity (a published stiffness value), the on-center spacing, the total load per square foot, and your deflection limit (L/360 is standard for floors; L/480-L/720 where more rigidity is wanted, such as under tile). This calculator computes a real span result for the US NDS 2018 method from your actual inputs; it does not publish one fixed span-table number here, because the correct answer always depends on which size, species, spacing and load you choose.

Australian Design Standard

Timber floor joists in Australia are governed by AS 1720 (Timber Structures) and span tables published against it, referenced through the National Construction Code (NCC) -- this calculator's Span mode currently computes results only for the US NDS 2018 method, so AS 1720 is selectable as your project's applicable standard but not yet populated with real Australian span data.

Pro Tips and Common Mistakes

At a typical material cost of around AUD 26 per joist, this calculator's default example -- a 3.66 m x 3.05 m floor with joists spaced 40.6 cm (16 in) on center, plus a 10% waste allowance -- works out to 10 floor joists and 2 end joists, or 12 pieces total. At a flat AUD 26 per piece, that is roughly AUD 312 in raw timber, before fasteners, joist hangers or flooring.

16 in on-center is the most common residential joist spacing in North America; 12 in OC suits heavier loads or longer spans, and 19.2 in or 24 in OC are used only with deeper joists or engineered I-joists rated for the wider spacing.
A deflection limit of L/360 is the common baseline for residential floors, but L/480 or stiffer is often specified under tile, stone or other rigid, crack-prone finishes, since those tolerate far less flex than carpet or wood flooring.
Double up joists under parallel partition walls and around stair, chimney or other floor openings -- a single joist is rarely sized to carry a concentrated wall or trimmer load on its own.
This calculator is a planning aid, not a stamped structural calculation. For anything beyond a simple shed, deck or non-habitable outbuilding floor, have your joist size, span and spacing confirmed against your design standard's official span tables, or by a licensed structural engineer, before you build.
A common mistake is comparing a nominal size (e.g. "2x10") against a competitor's span table without checking it uses the same dressed dimensions, species, grade and deflection limit -- those assumptions change the safe span, sometimes by several feet.