Calculator
Example Data Table
| Scenario | Joist span | Loads | Beam | Footing | Use |
|---|---|---|---|---|---|
| Small deck | 10 ft | 40 psf live, 10 psf dead | 2-ply 2x8 | 16 in round | Light residential layout |
| Standard deck | 12 ft | 40 psf live, 10 psf dead | 2-ply 2x10 | 18 in round | Common planning case |
| Heavy layout | 14 ft | 60 psf live, 15 psf dead | 3-ply 2x10 | 24 in round | Higher load review |
Formula Used
Factored surface load: q = (live load + dead load) × load factor.
Tributary width: T = joist span ÷ 2 + cantilever.
Line load on beam: w = q × T.
Beam section modulus: S = b × d² ÷ 6.
Beam moment of inertia: I = b × d³ ÷ 12.
Bending span: L = √((Fb × S × 8) ÷ (12 × w)).
Shear span: L = (2 × Fv × A) ÷ (1.5 × w).
Deflection span: L = ³√((384 × E × I) ÷ (5 × w × limit)).
Post span: L = adjusted post capacity ÷ (q × T).
Footing span: L = footing bearing capacity ÷ (q × T).
The calculator recommends the smallest span from these checks.
How To Use This Calculator
Choose the unit system first. Enter the deck beam line length, joist span, cantilever, and design loads. Add beam size, material stress values, post dimensions, post height, footing size, and soil bearing. Press the calculate button. Review the controlling limit. Use a final spacing lower than the recommended maximum.
Deck Post Span Planning Guide
Why Post Span Matters
Deck post span is the distance between support posts along a beam. This distance controls how much load each post, footing, and beam section must carry. A wide span can save material, but it also increases bending, shear, and deflection. A narrow span usually feels stiffer and gives better reserve capacity. Good planning balances comfort, safety, cost, and site limits before holes are dug.
Loads And Tributary Width
Every post supports a share of the deck surface. The calculator uses live load, dead load, joist span, and cantilever length to estimate the tributary width carried by the beam. Live load represents people, furniture, snow, or temporary use. Dead load represents framing, decking, railings, and fixed materials. Higher loads reduce the recommended span because each post must support more weight.
Beam Strength Checks
The beam is checked for bending, shear, and deflection. Bending checks the stress caused by the beam sagging between posts. Shear checks the vertical force near each post. Deflection checks visible movement under load. The smallest passing span becomes the beam limit. This conservative method helps users see which condition controls the design.
Post And Footing Checks
Posts must also carry axial compression. Tall posts may need bracing because slender members can lose strength. The footing check compares estimated post load against soil bearing area. Larger footings or stronger soil allow more capacity. Weak soil, small footings, or high decks can quickly reduce the spacing.
Practical Use
Use this tool during early layout planning. Enter values from lumber tables, local rules, or an engineer. Compare several beam sizes and footing diameters. Then choose a spacing lower than the calculated maximum. Always follow local building codes. Decks can fail when connectors, ledgers, lateral bracing, guards, or soil conditions are ignored. Treat this calculator as a planning aid, not a permit approval.
Design Notes
Check actual lumber sizes, not nominal labels. Moisture, species, grade, and treatment can change values. Keep posts plumb and seated on approved connectors. Recheck the layout after stairs, hot tubs, roofs, or heavy planters are added, because concentrated loads need special review. Record assumptions clearly, so later changes can be reviewed with confidence by builders and inspectors.
FAQs
What is deck post span?
Deck post span is the distance between posts along a supporting beam. It affects beam stress, footing load, post compression, and deck stiffness.
Can this calculator replace local building codes?
No. It is a planning tool. Always verify your deck layout with local code tables, permit rules, and qualified professionals when required.
Why does joist span affect post spacing?
Joist span changes tributary width. A longer joist span sends more deck area load to the beam, which reduces allowed post spacing.
What does the controlling limit mean?
The controlling limit is the smallest passing span from beam bending, shear, deflection, post compression, footing bearing, or your planning cap.
Why include footing bearing?
A strong beam still needs soil support. Small footings or weak soil can control post spacing before beam strength becomes critical.
What is a good deflection limit?
L/360 is common for many floor-like checks. Some decks may need stricter limits. Use the value required by your local code.
Should I use nominal or actual lumber size?
Use actual dimensions. For example, many 2x10 boards are about 1.5 inches thick and 9.25 inches deep.
Why add a load factor?
A load factor increases design load for a conservative check. Use 1.0 for service-level planning, or a higher value when appropriate.