Steel Beam Span Chart: Rules of Thumb by Size
As a rough budgeting rule, a steel floor beam's depth in inches is about its span in feet divided by 1.5 to 2, which is a span-to-depth ratio of roughly 18 to 24. By that rule a W12 lands around an 18 to 24 ft span and a W18 around 27 to 36 ft. Use it to budget and sanity-check drawings, never to size a beam.
Read this first: this is not a design table. Every span range on this page is an estimating rule of thumb. Real beam size depends on loads, deflection limits, bracing, vibration, connections and the building code, and it must be chosen by a licensed engineer of record. Don't buy, cut or install a beam based on this chart. Use it to budget, to check whether a drawing looks reasonable, or to know roughly what you're pricing before the drawings arrive.
With that said, here's how estimators actually use span-to-depth ratios, and where they break down.
How far can a steel beam span?
There's no single answer, because the same W12X26 can be perfectly fine at 24 ft carrying a light roof and badly undersized at 16 ft under a heavy floor with a wall on top. What estimators use instead is a depth-to-span ratio: for a typical, uniformly loaded floor beam, depth tends to fall in a predictable band relative to span.
The math is simple. If span-to-depth (L/d) is about 20:
- Depth in inches = span in feet x 12 / 20 = span in feet x 0.6
- Span in feet = depth in inches x 20 / 12 = depth in inches x 1.67
So a 20 ft span suggests something around 12" deep. A 30 ft span suggests something around 18" deep. That's the whole rule.
Experienced estimators carry a band instead of a single number: shallower and heavier members for heavy loads, deeper and lighter ones where deflection or vibration governs. Our conceptual estimating rules of thumb post covers how this fits with tonnage-per-square-foot budgeting.
What depth-to-span ratios are typical for floors and roofs?
These are estimating ranges, not code requirements.
| Member type | Typical L/d range for budgeting | Rough depth rule | Notes |
|---|---|---|---|
| Floor beam, typical office or retail | 18 to 22 | Depth (in) ≈ span (ft) x 0.55 to 0.67 | Vibration and deflection often govern |
| Floor girder carrying beams | 15 to 20 | Deeper than the beams it supports | Point loads from beams; usually heavier |
| Roof beam, light roof | 20 to 24 | Depth (in) ≈ span (ft) x 0.5 to 0.6 | Snow, ponding and equipment loads change this |
| Lintel or header over an opening | Varies widely | Load from above controls | Often a W8 to W12 or an angle; check the engineer's size |
| Residential flush beam | Limited by floor depth | Must fit in the joist depth | Often heavier than the ratio suggests |
Two cautions. First, a residential flush beam has to fit inside the floor depth, so the engineer often picks a heavier, shallower section than the ratio would suggest. Second, transfer beams carrying columns or walls have nothing to do with this table.
If you'd rather skip the hand lookup once the drawings land, upload the set to SteelFlo and get every beam the engineer sized in a BOM with weights. The first 3 takeoffs are free.
What are rough span ranges for W8, W10, W12, W16, W18 and W24 beams?
Here's the rule applied to common W shape depths. Nominal depth is used. These are for budgeting a typical uniformly loaded beam only.
| Nominal size | Depth (in) | Floor beam, L/d 18 to 22 | Roof beam, L/d 20 to 24 | Common light weights (lb/ft) |
|---|---|---|---|---|
| W8 | 8 | about 12 to 15 ft | about 13 to 16 ft | W8X10, W8X15, W8X18 |
| W10 | 10 | about 15 to 18 ft | about 17 to 20 ft | W10X12, W10X22, W10X26 |
| W12 | 12 | about 18 to 22 ft | about 20 to 24 ft | W12X14, W12X26 |
| W14 | 14 | about 21 to 26 ft | about 23 to 28 ft | W14X22 |
| W16 | 16 | about 24 to 29 ft | about 27 to 32 ft | W16X26, W16X31 |
| W18 | 18 | about 27 to 33 ft | about 30 to 36 ft | W18X35 |
| W21 | 21 | about 32 to 38 ft | about 35 to 42 ft | W21X44 |
| W24 | 24 | about 36 to 44 ft | about 40 to 48 ft | W24X55, W24X68 |
How to read a row: a W12 floor beam on a typical job is often somewhere around 18 to 22 ft. If you see a W12 spanning 35 ft on a floor, that's worth a second look. It may be right (heavy W12 with a light load, or a beam with intermediate support you missed), but it's unusual enough to check.
Same family, different weight: a W12X14 and a W12X26 are both about 12" deep, but the heavier one is much stronger and stiffer. The ratio tells you roughly what depth to expect, not which weight. That's why the weight column above is just examples, not recommendations. For the full W12 family see our W12 beam sizes chart, and for every W depth see the wide flange beam sizes guide.
Common questions this answers (roughly)
- "What size steel beam for a 20 foot span?" By the rule, something around 11" to 13" deep, so a W10 or W12 is a reasonable budget guess for a typical floor. Load can easily push it deeper or heavier.
- "How far can a W8 span?" For a typical floor beam, around 12 to 15 ft by the rule. A residential engineer may still pick a W8 at a different span because of floor depth or load.
- "How far can a W12 span?" For budgeting, around 18 to 24 ft depending on floor vs roof.
All three answers end the same way: the engineer decides.
Why do load, deflection and bracing change everything?
The ratio works because typical buildings have typical loads. Change any of these and it stops working.
Load. A beam carrying a bearing wall, a column, a heavy mechanical unit or storage loads can need a much bigger section than the ratio suggests. Concentrated loads hurt more than uniform ones.
Deflection. Building codes limit how much a beam can sag. A common floor live-load limit is span/360. AISC's preliminary beam, girder and column size tables, written for architects at early design, use that same span/360 live-load limit along with 50 ksi steel. Stiffer limits, such as under brittle finishes, push depth up.
Vibration. Long, shallow floor beams can feel bouncy even when they're strong enough. Vibration is a big reason floor beams don't get shallower than roughly span/20 on many office and residential floors.
Lateral bracing. A beam with its compression flange braced by a deck or joists can carry much more than the same beam unbraced. An unbraced beam in a garage or a long header can need a heavier section.
Composite action. Beams with shear studs acting with a concrete slab can be shallower for the same span. You'll see this on commercial floors.
Continuity and cantilevers. Continuous beams and cantilevers behave differently from simple spans.
Steel grade. Most W shapes today are ASTM A992 at 50 ksi. Older buildings may have weaker steel.
The engineer works all of this through the AISC Specification (ANSI/AISC 360-22) and the load tables in the 16th edition Steel Construction Manual. Those are the real sources, and they need an engineer to apply them correctly.
When do you need an engineer?
Always, for construction. Every beam that will be built needs to be sized by a licensed engineer, and most building departments will require stamped drawings for a permit. That's true for a commercial floor and it's true for a homeowner taking out a bearing wall.
Where estimators can use the ratio:
- Conceptual budgets before structural drawings exist.
- Sanity checks on drawings, to catch a possible typo (a W12X16 shown spanning 40 ft) and send an RFI.
- Early tonnage guesses for a design-build pursuit.
Where they shouldn't:
- Telling a homeowner what beam to buy.
- Substituting a different size than the engineer specified.
- Pricing a firm bid off a guess when drawings are coming.
If a customer asks you to size a beam, the right answer is to refer them to an engineer. That protects them and protects you.
How do you budget the beam once it's sized?
Once the engineer gives you a size, pricing is straightforward. Start with weight:
Piece weight (lb) = weight per foot (lb/ft) x length (ft)
A W12X26 at 22 ft weighs 26 x 22 = 572 lb. Add bearing length on each end if the drawing gives clear span only. Our guide on how to calculate steel beam weight walks through more examples, and the W12X26 shape page has the dimensions.
| Example | Size | Length | lb/ft | Piece weight |
|---|---|---|---|---|
| Residential wall removal | W8X18 | 15 ft | 18 | 270 lb |
| Garage header | W10X22 | 18 ft | 22 | 396 lb |
| Light commercial floor beam | W12X26 | 22 ft | 26 | 572 lb |
| Office floor beam | W16X26 | 28 ft | 26 | 728 lb |
| Long-span floor beam | W18X35 | 32 ft | 35 | 1,120 lb |
| Long-span roof girder | W24X55 | 42 ft | 55 | 2,310 lb |
Notice the W16X26 weighs the same per foot as the W12X26 but spans further at typical loads, because depth is what buys stiffness. That's why engineers often pick a deeper, lighter section when floor depth allows.
Then build the price: material at your current rate, fabrication (cut, drill, connections, any cap or bearing plates), finish, delivery and erection. Plug the weight into the steel weight calculator for a fast material number. For whole-building budgets, our steel per square foot rules of thumb give tonnage ranges by building type.
Frequently Asked Questions
What size steel beam do I need for a 20 foot span?
Only an engineer can tell you for an actual project. As an estimating rule of thumb, a typical floor beam at 20 ft is often around 11" to 13" deep, so a W10 or W12 is a common budget assumption. Heavy loads, point loads or tight deflection limits can change it a lot.
How far can a W8 steel beam span?
For budgeting a typical floor, a span-to-depth ratio of about 18 to 22 suggests roughly 12 to 15 ft. The real limit depends on the W8's weight, the load and bracing. An engineer has to confirm it.
What is a good span-to-depth ratio for steel beams?
For typical floor beams, estimators often use about 18 to 22, and about 20 to 24 for lightly loaded roof beams. These are rules of thumb for budgeting. Design ratios come from the engineer's calculations for strength, deflection and vibration.
Can I use a span table to buy a steel beam?
No. Online span tables, including this one, aren't a substitute for an engineer's design. Loads, bracing, connections and code requirements vary by building, and most permits require stamped drawings.
Is a deeper beam always better?
A deeper beam is usually stiffer per pound, which is why engineers prefer depth when the floor allows it. But depth costs headroom and may not fit the floor system. The engineer balances depth, weight and fit.
Bottom line
Span-to-depth rules are good for budgets and for catching drawings that look off, and that's all they're for. Once the engineer sizes the beam, the estimating work is weight, fabrication and labor. Price a single beam in the free calculator, or upload the full drawing set to SteelFlo to get every sized member in a BOM you can price with your own rates.