You can estimate steel tonnage early in three ways, depending on how far the design has gone: from comparable past projects at concept stage, by sizing a typical structural bay and multiplying it out once the column grid is known, and from a schematic member list once preliminary framing plans exist. Each one gives tonnage for the main frame, and you then add connections, base plates, bracing and miscellaneous steel. The estimate gets tighter at every stage, and a fabricator's take-off of issued drawings replaces it in the end.
This article is about quantity: kilograms and tonnes. For how tonnage turns into a budget and why area-based cost figures mislead, see our article on structural steel cost per square foot.
Why estimate tonnage before cost
Steel prices change; the amount of steel in a design doesn't, unless the design changes. That makes tonnage the stable number to estimate first:
- It can be checked. A tonnage figure can be compared with the next estimate, with the engineer's numbers and with a fabricator's take-off. A lump sum can't be broken down the same way.
- It separates quantity from price. If the budget moves, you can tell whether the building got heavier or the market got more expensive.
- It drives other decisions. Tonnage affects foundation loads, crane selection, delivery planning and the erection schedule.
The trap is false precision. An early tonnage is an estimate with a range. Write it down with the assumptions behind it (grid, loads, storeys, lateral system), so when the number moves you know why.
Method 1: comparable projects (concept stage)
Before there is a column grid, the only real basis is history: how much steel went into similar buildings.
How estimators do it:
- Find genuinely comparable projects. Same use, similar spans, similar number of storeys, similar roof and floor loads, same kind of lateral system, similar location for snow.
- Divide their final steel tonnage by floor area to get a steel intensity (kg/m²) for each.
- Apply it to the new building's area, adjusted for known differences.
The quality of this method depends entirely on the quality of the comparables. That is why we don't publish generic kg/m² figures: a number from a different building, span or snow region can be far off for yours. If you don't have your own project records, ask the structural engineer for a preliminary intensity for your layout, or ask a fabricator to price an early scheme.
Building your own database. If you build or design steel buildings regularly, keep a simple record for each completed project: use, floor area, storeys, typical bay size, roof type, lateral system, location, and the final steel tonnage split into main members, connections and miscellaneous steel. After a few projects, your concept estimates get much better.
Method 2: the typical bay (once the grid is known)
Once the architect has a column grid and the engineer has preliminary member sizes, you can estimate far more precisely. Most buildings repeat a typical bay: the area between four columns, with the same beams and girders in each.
The steps:
- Take one typical bay and list the members in it: infill beams, girders, and the column (each interior column supports one bay's worth of floor per level).
- Get preliminary sizes from the engineer, or from preliminary design.
- Multiply each member's length by its mass per metre.
- Multiply the bay weight by the number of bays, per floor.
- Treat edge bays, openings, roof framing and special areas separately.
Worked example (the sizes below are placeholders to show the arithmetic, not a design and not a benchmark):
A floor bay 9.0 m by 7.5 m, with infill beams at 2.5 m spacing spanning 9.0 m onto girders that span 7.5 m between columns, and a 4.0 m storey height.
| Member | Quantity per bay | Assumed size | Calculation | Weight |
|---|---|---|---|---|
| Infill beams | 3 at 9.0 m | W410x46 (46.2 kg/m) | 3 x 9.0 x 46.2 | 1,247 kg |
| Girder | 1 at 7.5 m | W530x74 (74.4 kg/m) | 7.5 x 74.4 | 558 kg |
| Column (one storey) | 1 at 4.0 m | W250x58 (58.2 kg/m) | 4.0 x 58.2 | 233 kg |
| Bay total | about 2,040 kg |
With 24 typical bays on a floor, that is about 49 tonnes of main framing for that floor, before edges, openings, connections and bracing. Change the girder or beam size, and you can see immediately what it does to the total. That is the real value of this method: it shows which decisions move the tonnage.
Weights per metre come from section tables; our guide on how to calculate structural steel weight covers the arithmetic for every shape.
Method 3: the schematic member list
When preliminary framing plans exist, even unchecked ones, you can count members directly:
- Go plan by plan and list every beam, girder, column, brace and lintel with its size and length.
- Use grid dimensions for lengths at this stage; precise cut lengths come later.
- Where sizes aren't shown yet, use the typical size for similar members and flag them as assumed.
This is essentially an early version of a full take-off. Our guide on how to do a structural steel takeoff walks through the full process. At schematic stage you won't have connection details, so the next section matters.
What to add on top of the main members
The main members are only part of the steel package. Early estimates commonly miss these:
| Item | What it covers | How to handle early |
|---|---|---|
| Connections | Shear tabs, clip angles, end plates, stiffeners, bolts | Add an allowance based on your own past projects and the connection type |
| Base plates and anchor rods | One base per column; bigger for moment bases | Count columns and estimate a typical plate |
| Bracing | Braces, gussets, collectors | Count braced bays from the lateral system concept |
| Roof framing | Purlins, girts, roof beams, frames around openings | Treat as its own area, not as a floor bay |
| Edge and opening framing | Slab edge angles, frames around stairs, shafts and rooftop units | List separately; often underestimated |
| Miscellaneous steel | Stairs, railings, ladders, lintels, supports | Usually a separate line item |
| Joists and deck | Open web steel joists and steel deck | Usually estimated and supplied separately |
Moment connections and complex geometry add fabrication hours more than they add weight, so don't judge a design's cost by tonnage alone.
What moves the tonnage most
When an early estimate changes, it is usually because one of these changed:
- Span and grid. Longer spans need deeper, heavier beams. The column grid is the biggest single lever.
- Loads. Storage, mezzanines, equipment and future expansion add steel. In Ontario, roof snow loads come from the climatic data in Supplementary Standard SB-1 of the Building Code, and drifts against higher roofs and parapets add more.
- Height. Taller storeys mean longer, heavier columns and more bracing.
- Lateral system. Moment frames generally need more steel than braced frames.
- Cranes and heavy equipment. Runway beams, brackets and heavier columns.
- Serviceability limits. Tight deflection or vibration limits can drive members up a size even when strength is fine.
Track these assumptions alongside the number, and each revision becomes easy to explain.
Carry a contingency that shrinks
Early estimates should carry a contingency for what isn't known yet, and that contingency should shrink as the design firms up:
- Concept: largest, because the grid, loads and system can all still change.
- Typical bay: smaller, once the grid and preliminary sizes are agreed.
- Schematic member list: smaller again, with connections still an allowance.
- Take-off of issued drawings: the estimate becomes a quantity, and the contingency covers revisions only.
The sizes of those contingencies are a judgement call for the estimator and owner, based on how settled the design is. Write them down separately rather than burying them in the tonnage.
Get a budget take-off early
The fastest way to tighten an early estimate is to have a fabricator look at it. We prepare detailed take-offs and quotes free of charge from structural or architectural drawings. If your drawings are still preliminary, tell us when you send them so the quote states its assumptions. Engineering services are also available for projects that need them. Our article on steel estimating services explains what a fabricator's estimate covers.
Send your drawings for a take-off and quote, or see our services for detailing, fabrication, delivery and installation.
Frequently asked questions
Is there a thumb rule for structural steel quantity?
Estimators do use steel weight per square metre of floor, but only from comparable buildings they have real data for. Generic figures copied from the internet ignore span, height, loads and the bracing system, so they can be badly off for your building. A typical bay calculation is a better early check.
How do you estimate the steel needed for a building?
At concept stage, use steel intensity from comparable past projects. Once the column grid is known, size a typical bay and multiply it out. Once schematic framing plans exist, count members and lengths. Then add connections, base plates, bracing and miscellaneous items.
How accurate is an early steel tonnage estimate?
It depends on how much design information exists. A concept estimate from comparable projects is a range, a typical bay estimate is tighter, and a member-by-member take-off from issued drawings is the most reliable. Carry a contingency that shrinks as the design develops.
What increases steel tonnage the most?
Longer spans and a wider column grid, heavier floor or roof loads including snow drifts, taller buildings, cranes and heavy equipment, and moment frames instead of braced frames all add tonnage.
