Bar joist calculator for count, weight & bridging
Enter the bay, span, spacing and joist weight per foot from the SJI load table for the designation on the drawings. The calculator counts joists, totals tons, sizes the bridging and gives the deck area the joists carry.
- Joists per bay
- Weight in tons
- Bridging rows
- Deck area served
1 · Bay
2 · Joist & bridging
Joist weight per foot and required bridging rows come from the Steel Joist Institute (SJI) load and bridging tables for the specified designation and span; the defaults here are placeholders to replace. Joist suppliers design and furnish joists to the specified loads.
Live formula
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Copy these results, then add your approved sample files in the trial brief.
What 18K5 or 24LH07 tells you
A joist designation is depth, series and chord size: 18K5 is an 18 in deep K-series joist with a size-5 chord. LH and DLH are long-span and deep long-span series. The designation alone does not give you weight. The Steel Joist Institute load tables list the approximate weight per foot for each designation, and the joist supplier designs and supplies the joists to the loads on the drawings. For estimating, take the weight per foot from the SJI table for the designation shown and confirm it against the supplier's quote.
- Spacing. Usually shown on the framing plan as "@ 5'-0" o.c." Count joists as bay length ÷ spacing + 1 per line, minus any lines carried by beams or walls.
- Span. Use the joist length from bearing to bearing, plus seat extensions if you are pricing by actual length.
- Bridging. Rows required by the SJI bridging tables for the span and chord size. Horizontal bridging runs continuously as two lines per row; diagonal bridging is counted in sets per joist space.
- Specials. KCS joists, joist girders and joists with extra loads from equipment or hanging loads are priced by the supplier; list them separately.
Counts, weight and bridging
| Item | Where it comes from |
|---|---|
| Designation | Framing plan, joist schedule |
| Weight per foot | SJI load table for the designation |
| Bridging rows | SJI bridging tables, span and chord size |
| Seat depth and extensions | SJI standard details, drawings |
| Special loads | Drawings, supplier design |
The default joist weight and bridging rows are placeholders. Replace them with the SJI table values for the designation on your drawings.
Where joists fit
Most steel fabricators buy joists and deck rather than fabricate them, so the joist line in the bid is a supplier quote plus erection. The estimator's job is to get the counts and lengths right so the quote is complete, and to carry erection: pieces, picks, bridging installation and bolting or welding of seats. Joist count is a direct driver of erection pieces, which is why a quick, checkable count per bay matters even when the joist supplier does the design.
RenderDraw's AI takeoff helps steel estimators pull those counts from framing plans: it proposes joist lines, spacing and bays with the source sheet attached and flags designations or spacing callouts it could not confirm.
Member counts off the drawings, with the sheet attached.
RenderDraw's AI takeoff proposes steel member counts and lengths from plans, framing plans and schedules, links every row to its source, and sends low-confidence items to your estimator before the handoff to pricing.
- Counts and lengths by member. Proposed from the drawings and kept tied to the framing plan or schedule they came from.
- Uncertain items flagged. Items the AI can't confirm go to review instead of being guessed.
- Addenda through the same review. Send the changed sheets through takeoff and review, then reprice the members that changed.
- Your estimator decides. Connections, erection, coatings, risk and margin stay your call.
Test it on a real steel package
Send one structural set, ideally with an addendum. Sample files stay private while RenderDraw assesses the request. No obligation.
RenderDraw does not publish accuracy percentages or time-saved claims. Test it on your own plan set.
Questions estimators ask.
How many joists do I need for a bay?
Divide the bay length by the spacing, round up and add one. A 40 ft bay at 5 ft spacing has 9 joist lines. Subtract the end lines if they sit on beams or walls.
How much does a bar joist weigh?
It depends on the designation. The SJI load tables give an approximate weight per foot for each designation. Multiply by the length and the count.
How many rows of bridging do joists need?
The SJI bridging tables set the number of rows from the span and the top chord size. Enter the rows from the table for your designation.
What does K-series mean?
K-series are open-web steel joists for typical floor and roof spans. LH and DLH are long-span series. The SJI publishes specifications and load tables for each.
Can RenderDraw count joists from a framing plan?
Yes, RenderDraw's AI takeoff proposes joist lines, spacing and counts from framing plans with sources and review flags. Request a free sample takeoff.
Framing plans, counted for you.
Send a steel set. RenderDraw proposes joist marks, spacing, spans and bay counts from framing plans with their source, and flags designations it could not read.
- Upload the plans, specs and schedules, plus an addendum if you have one.
- RenderDraw reviews the set and the scope you want tested.
- We follow up with extracted quantities, flagged items and next steps.