How to use this calculator
- Choose joist size. Pick a common dressed 2x joist size or enter actual custom width and depth.
- Enter spacing and deck loads. Use on-center joist spacing plus live and dead area loads in psf or kPa.
- Enter adjusted design values. Replace the example E, Fb and Fv values with values for the actual species, grade, size and deck conditions.
- Set deflection limits. Use live and total deflection limits from the governing design basis when appropriate.
- Read the governing span. Compare bending, shear, live-deflection and total-deflection spans, then review utilization at the entered check span.
How it works
This deck joist span calculator screens a single sawn joist as a simply supported rectangular member under uniform deck load. It starts by converting live and dead area loads into line loads from joist spacing:
w_L = q_L x spacing
w = (q_L + q_D) x spacing
For actual joist width b and depth d, the rectangular
section properties are:
I = b x d^3 / 12
S = b x d^2 / 6
Deck Joist Span Worksheet
Use this worksheet to translate a deck joist span calculator or span chart search into the live inputs. It screens an entered joist and design-value basis; it does not replace an adopted span table or permit review.
| Field question | Calculator input | Design read |
|---|---|---|
| What joist are you checking? | Joist size or custom width/depth | Sets b, d, I and S for bending, shear and deflection. |
| How far apart are joists? | Joist spacing | Converts deck area load into line load carried by one joist. |
| What load is on the deck? | Live area load and dead area load | Creates live and total line loads for the uniform-load checks. |
| What design values apply? | E, Fb and Fv | Use adjusted values for actual species, grade, treatment, wet-service condition and design basis. |
| What serviceability limit applies? | Live and total deflection limits | Controls the L/n back-solved span limits. |
| What span do you want to verify? | Deck joist span to check | Reports utilization at the entered check span against the governing mode. |
| Is the search really about an opening? | Lintel/header workflow | Opening width or wall height belongs to the lintel-size calculator, not deck joist span. |
The simple-span uniform-load checks use:
M = w x L^2 / 8
V = w x L / 2
fb = M / S
fv = 1.5 x V / (b x d)
delta = 5 x w x L^4 / (384 x E x I)
The shortest back-solved span is reported as the maximum joist span for the entered assumptions, with the checked-span utilization shown beside it. For species-led query wording, use the joist span calculator by species. For spacing-led deck searches, use the deck joist span calculator by spacing and species. Searches that mention opening width or wall height are often really asking for a header or lintel over an opening; use the lintel size calculator for that load path.
Worked example
Verified against the live calculator
A 2x10 deck joist at 16 in on-center with example
loads of 40 psf live plus 10 psf dead produces a
total line load of 66.7 lb/ft. With example values
E = 1.2 Mpsi, Fb = 1000 psi and
Fv = 135 psi, the calculator compares bending, shear, live
deflection and total deflection, then reports the governing span and the
utilization at a 12 ft check span.
Frequently asked questions
How do you calculate deck joist span?
This screen converts deck live and dead area loads into a uniform line load by multiplying by joist spacing. It then back-solves simple-span limits for bending stress, shear stress, live-load deflection and total-load deflection and reports the shortest governing span.
Is this an IRC, AWC or permit span table?
No. Span tables include species, grade, use condition, adjustment factors, code assumptions and local adoption rules. This page uses the E, Fb and Fv values you enter, so it is a transparent engineering screen rather than an official prescriptive selector.
What values should I use for E, Fb and Fv?
Use adjusted design values for the actual joist species, grade, size, preservative treatment, wet-service condition, duration and design edition. Do not mix design values from one table with code assumptions from another source.
Does joist spacing affect span?
Yes. The joist tributary width is the on-center spacing. A 24 in spacing carries 50% more line load than a 16 in spacing for the same psf area load.
Is opening width or wall height part of deck joist span?
Not usually. Deck joist span is the clear span between joist supports, controlled by joist size, spacing, loads and wood design values. If the question is an opening in a wall or masonry veneer with tributary wall height, use the lintel size calculator instead.
Does it handle cantilevered deck joists?
No. This page is for a simple span between supports. Cantilevers, backspans, multi-span continuity, ledger attachment, beam drops and guard loads need code tables or engineered design.
Can I use this for a hot tub deck?
Not by itself. Hot tubs, planters, masonry, concentrated loads and other heavy features need project-specific load paths, joist/beam/footing design and connection checks.
Method & assumptions
- Simple-span, single rectangular sawn deck joist with uniform load only.
- Common 2x sizes use dressed North American actual dimensions; custom mode uses the actual dimensions you enter.
- Area loads are multiplied by joist spacing to create uniform line load. Verify the tributary width and load path.
- Defaults for load, E, Fb and Fv are examples only. Use adjusted values from the adopted design standard, actual species, grade and deck conditions.
- Does not include span tables, local code amendments, preservative-treatment or incising adjustments, wet-service factors, repetitive-member factors, bearing, notches, holes, lateral restraint, ledgers, hangers, fasteners, guards, stairs, hot tubs, snow drift, wind uplift, seismic load combinations or permit review.
- Use deck board takeoff, deck footing concrete, wood beam span, lintel size and rafter layout for adjacent planning checks.