Glulam Beam Size Calculator
Screen a simply supported glulam beam by span, tributary load, point load, grade, width, deflection limit, bearing length, and service factors.
Calculation Breakdown
| Grade | Typical Use | Fb, psi | E, psi | Notes |
|---|---|---|---|---|
| 24F-V4 | Floor beams | 2400 | 1,800,000 | Balanced Douglas-fir screening grade |
| 24F-V8 | Longer spans | 2400 | 1,900,000 | Higher tension lamination combination |
| 20F-V12 | Southern pine | 2000 | 1,700,000 | Common SP framing combination |
| 20F-E | Eastern species | 2000 | 1,600,000 | Use supplier stamp for final values |
| 18F-V3 | Utility beams | 1800 | 1,500,000 | Shorter spans and lighter loads |
| 16F-V3 | Light framing | 1600 | 1,400,000 | Often controlled by deflection |
| 30F-E2 | Industrial beams | 3000 | 2,100,000 | Premium grade, verify availability |
| Nominal Depth | Actual Depth | Typical Widths | Use Range | Screening Note |
|---|---|---|---|---|
| 7-1/4 in | 7.25 in | 3.125, 3.5 | Headers | Short openings and light roofs |
| 9-1/2 in | 9.5 in | 3.5, 5.125 | Decks | Often fits shallow framing |
| 11-7/8 in | 11.875 in | 3.5, 5.5 | Floors | Common for moderate residential spans |
| 13-1/2 in | 13.5 in | 3.5, 5.5 | Long floors | Good first trial for 16 to 18 ft |
| 16 in | 16 in | 5.125, 6.75 | Girders | Useful where deflection controls |
| 18 in | 18 in | 5.125, 6.75 | Ridge beams | Check lateral bracing and bearing |
| 21 in | 21 in | 5.5, 6.75 | Heavy beams | Reactions may control support design |
| 24 in+ | 24 to 36 in | 6.75, 8.75 | Large spans | Engineer connection and camber details |
| Application | Common Live Load | Common Dead Load | Deflection Limit | Important Check |
|---|---|---|---|---|
| Bedroom floor | 30 psf | 10 psf | L/360 | Vibration feel and finish stiffness |
| Living floor | 40 psf | 10 to 15 psf | L/360 | Deflection and bearing reaction |
| Deck beam | 40 to 60 psf | 10 psf | L/360 | Wet service and connector capacity |
| Snow roof | 20 to 70 psf | 10 to 20 psf | L/240 | Snow drift and duration factor |
| Ridge beam | Project specific | 10 to 20 psf | L/240+ | Rafter thrust, posts, and foundations |
| Tile floor | 40 psf | 15 to 20 psf | L/480 | Finish cracking and subfloor stiffness |
| Check | Formula Used | Units | Pass Basis |
|---|---|---|---|
| Uniform line load | w = area load x tributary width | plf | Service load converted to beam line load |
| Max moment | M = wL²/8 + Pab/L | lb-in | Demand less than adjusted Fb x S |
| Max shear | V = reaction at controlling support | lb | 1.5V/bd less than adjusted Fv |
| Deflection | 5wL⁴/384EI plus centered P estimate | in | Actual deflection less than L/limit |
| Bearing | fc = reaction / bearing area | psi | Stress less than perpendicular compression value |
It starts out as a napkin sketch and ends up as a stamped set of structural plans. In between, it gets dicey, as homeowners grapple with figuring out how big a beam should be to support a heavy roof or second floor. One reason glulams look so nice is they’re predictable. This predictability depends on strict engineering rules that seem odd. For instance, thicker doesn’t necessarily mean stronger (at least, not in the intuitive manner you might think). Longer spans don’t require proportionately deeper members. And soon, the numbers gets funky.
So by narrowing down your choices beforehand, you save yourself both time and money when you consult an engineer. The key is to first get your load path correct. It isn’t just the weight of wood that you’re supporting. It’s all the stuff above it (drywall, flooring, furniture etc.) as well as people jumping on a trampoline. Once you’ve defined the tributary width (the area of roof or floor which transfers its weight onto your given beam), then the calculator do the math for you.
How to Choose the Right Size Beam
A lot of DIYers use the entire width of their room, whereas they really should of being counting half the span coming in from either side. This is a small detail but it makes a significant difference in what size beam you have to get. Underestimating the load? Beam sags. Overestimating it? Waste money on material you don’t need to build.
In residential jobs, the main constraint is typically deflection. You want strength, sure, but who wants a bouncing bouncy floor where you feel like you’re walking on a trampoline and your tiles will crack? Enter the deflection limit. With a standard floor, they use an L/360 ratio, which means the floor can only deflect less than an inch under a load across a 12 foot span. If you’ve got drywall hung from the ceiling underneath, that may have to be tightened up so as not to cause cracks. You can also dial back up the sensitivity of this with the tool. It’s not just about safety, it’s also about comfort and the final look of the room.
Then there’s the matter of grade. Glulams are not all alike. Although two might appear similar in yard, a 16F-V3 beam will not have the same strength properties as a 24F-V4. This refers to its base grade: higher grade beams bends and flex stiffer, allowing longer spans without as deep a beam. But those are also pricier and perhaps more difficult to find locally. Your options come down to your budget versus your maximum ceiling height allowance. Less headroom means a deeper beam, which is a real-world tradeoff you’ll experience daily.
Hardware and theory create bearings and connections. Even though the center of a beam can be very strong, if it isn’t firmly seated on the supports then what good is it? That’s where bearing comes into play. If the beam is not properly supported by long enough bearings, it will crush the wood fibers at the end. It should transfer the load cleanly into whatever it sits upon (whether that’s a post or another wall). The calculator automatically takes that into account from the reaction forces, as those are largest at the supports.
Moisture matters too. A beam installed within a climate-controlled space acts different compared to one supporting an overhang outside on a deck subject to temperature fluctuations and humidity. Harsh service conditions reduce the allowable stress values, i.e., weakening the wood.
You must give special consideration to point loads. Perhaps there’s a post coming down from your upper floor landings right onto the span, or maybe you’ve got a really heavy chandelier suspended in mid-air. Focused forces like these will result in different types of bend patterns different than uniform loads. With the tool, instead of spreading point loads across the entire span, you can put them precisely where they fall. Why? That’s typically where peak stress occurs.
The bottom line: This is a screening device, not a permit. It’s a way to narrow down options without wasting your engineer on impractical requests. Ultimately, any design must be checked against vibration limits, lateral stability, and all the details of local code. That can’t be captured in a generic tool. Narrow the list with it, then show it to a pro. The idea is to have some informed questions going into engineering stage instead of a bunch of blank stares.
When done right, a well-sized beam vanishes into the architecture. It simply holds things up (quietly), reliably, so you can completely forget about it.
