How Much Weight Can Wood Hold Calculator
Estimate both the breaking strength and the sag (deflection) of a wood shelf or beam — because a board usually sags too much long before it breaks.
Quick Answer
Wood load capacity has two limits: strength (when it breaks) and stiffness (when it sags too much). Sag usually governs. The safe load is the lower of the breaking load and the load that keeps sag within the L/360 limit (span / 360). Thicker boards and shorter spans help most.
Wood Load & Sag Calculator
Enter the span, board size, species, and the weight you plan to put on it. The calculator gives the breaking load, the load at the L/360 sag limit, and the sag at your load.
How to Use This Calculator
Strength & Sag Formulas
Section modulus S = b x d² / 6 | Moment of inertia I = b x d³ / 12
Breaking load = k x Fb x S / span (k = 8 distributed, 4 point)
Sag = 5 x load x span³ / (384 x E x I) (distributed)
Sag limit = span / 360
Strength vs Sag — Reference
| Wood type | Bending stress (Fb) | Stiffness (E) |
|---|---|---|
| Pine / SPF | ~875 psi | ~1.3 million psi |
| Douglas Fir | ~1,000 psi | ~1.7 million psi |
| Plywood | ~1,000 psi | ~1.5 million psi |
| Hardwood (oak/maple) | ~1,400 psi | ~1.8 million psi |
Common Mistakes to Avoid
- Checking only breaking strength and ignoring sag — sag usually governs.
- Laying a board flat instead of on edge; depth drives both strength and stiffness.
- Forgetting that wood creeps and sags up to ~50% more over time.
- Loading to the calculated maximum with no safety margin.
When the Estimate May Be Wrong
This is a simplified single-beam estimate. Real capacity depends on grade, knots, moisture, load duration, fasteners, and long-term creep. It does not replace engineering. For floors, decks, stairs, or any load-bearing member, verify with code-approved span tables or a qualified engineer.
Wood Load & Sag FAQs
How much weight can a wood shelf hold?
It is limited by two things: strength (when it breaks) and stiffness (when it sags too much). Usually sag is the real limit. Enter your span, board size, and species above to see both the breaking load and the load at which sag reaches the L/360 standard.
How much will my shelf sag under load?
Sag depends on load, span, board stiffness (modulus of elasticity), and the moment of inertia (width x thickness cubed / 12). The calculator shows the sag at your planned load and compares it to the L/360 limit. Wood also creeps, sagging up to ~50% more over time.
What is the L/360 sag rule?
L/360 is the common deflection limit: maximum acceptable sag equals the span divided by 360. A 36-inch shelf should sag no more than 0.10 inch. Many woodworkers target an even tighter 0.02 inch per foot to allow for long-term creep.
How do you make a shelf hold more weight?
Increase thickness first — capacity rises with the cube of depth, so going from 3/4 to 1.5 inch is about 8x stiffer. Shorten the span or add a center support, choose a stiffer species or plywood, or add a front edge/apron to act as a beam.
Is this wood load calculator safe for structural use?
No. Treat it as an educational planning estimate for shelves and light beams. Grade, defects, moisture, connections, and load duration all matter. For floors, decks, stairs, or load-bearing members, verify with code-approved span tables or an engineer.
Sources and Methodology
This is an original Woodworking Advisor calculator guide combining standard beam bending and deflection formulas with conservative assumptions and clear limits.
- Bending: S = bd²/6, breaking load = kFbS/L. Deflection: I = bd³/12, sag = 5WL³/(384EI) distributed or WL³/(48EI) point; limit L/360.
- Fb and E are typical design ranges; actual values vary by species and grade (set by the NDS and codes).
- Not a substitute for engineered span tables; safety-critical loads must be verified professionally.
