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40 Formulas

Construction Formulas

Span, load, coverage and installation equations.

Floor Span Calculator

Open the Floor Span Calculator →
Floor Span Calculator
Line load w = Total load (kN/m2) x Spacing (m)
Used in Floor Span Calculator
Floor Span Calculator
Second moment of area I = b x d^3 / 12
Used in Floor Span Calculator
Floor Span Calculator
Deflection (UDL) = 5 w L^4 / (384 E I)
Used in Floor Span Calculator
Floor Span Calculator
Bending moment M = w L^2 / 8, stress = M / Z (Z = b d^2/6)
Used in Floor Span Calculator
Floor Span Calculator
Pass if deflection <= L/300 AND stress <= grade strength
Used in Floor Span Calculator

Flooring Coverage Calculator

Open the Flooring Coverage Calculator →
Flooring Coverage Calculator
Floor area = (Room length x Room width) in m2
Used in Flooring Coverage Calculator
Flooring Coverage Calculator
Area needed = Floor area x (1 + waste%)
Used in Flooring Coverage Calculator
Flooring Coverage Calculator
Plank area = Plank length x Plank width
Used in Flooring Coverage Calculator
Flooring Coverage Calculator
Planks = ceil(Area needed / Plank area)
Used in Flooring Coverage Calculator
Flooring Coverage Calculator
Packs = ceil(Area needed / Pack coverage)
Used in Flooring Coverage Calculator

Sanding Sequence Planner

Open the Sanding Sequence Planner →
Progress through grits
80 -> 120 -> 150 -> 180 -> 220 ...
Used in Sanding Sequence Planner
Sanding Sequence Planner
Never jump more than ~one grade (avoid over ~1.7x increase)
Used in Sanding Sequence Planner
Sanding Sequence Planner
Each grit removes the scratches of the previous grit
Used in Sanding Sequence Planner
Sanding Sequence Planner
Always sand with the grain, not across it
Used in Sanding Sequence Planner
Sanding Sequence Planner
Remove all dust between grits to avoid stray coarse scratches
Used in Sanding Sequence Planner

Timber Post Calculator

Open the Timber Post Calculator →
Timber Post Calculator
Effective length Le = K x Height (K by end condition)
Used in Timber Post Calculator
Timber Post Calculator
Slenderness = Le / radius of gyration (r = sqrt(I/A))
Used in Timber Post Calculator
Timber Post Calculator
Compression (squash) capacity = fc x Area
Used in Timber Post Calculator
Timber Post Calculator
Euler buckling load Pcr = pi squared x E x I / Le squared
Used in Timber Post Calculator
Timber Post Calculator
Safe if capacity >= load AND slenderness <= 180
Used in Timber Post Calculator

Wind Load Calculator

Open the Wind Load Calculator →
Wind Load Calculator
Dynamic pressure q = 0.613 x V squared (V in m/s, q in Pa)
Used in Wind Load Calculator
Wind Load Calculator
Design pressure = q x Terrain factor x Height factor
Used in Wind Load Calculator
Wind Load Calculator
Height factor = (Height / 10) ^ 0.14 (above 10m)
Used in Wind Load Calculator
Wind Load Calculator
Wind force = Design pressure x Area x Force coefficient
Used in Wind Load Calculator
Wind Load Calculator
1 kN ~ 102 kgf (divide Newtons by 9.81)
Used in Wind Load Calculator

Glue Consumption Tool

Open the Glue Consumption Tool →
Glue Consumption Tool
Total bond area = Area per joint x Number of joints/faces
Used in Glue Consumption Tool
Glue Consumption Tool
Glue (grams) = Total area x Spread rate x (1 + waste%)
Used in Glue Consumption Tool
Glue Consumption Tool
Kilograms = Grams / 1000
Used in Glue Consumption Tool
Glue Consumption Tool
Packs = ceil(Kilograms / Pack size)
Used in Glue Consumption Tool
Typical spreads
PVA 150-250, PUR 120-180, epoxy 200-400 g/m2
Used in Glue Consumption Tool

Stair Geometry Tool

Open the Stair Geometry Tool →
Stair Geometry Tool
Number of risers = round(Total rise / Target riser)
Used in Stair Geometry Tool
Stair Geometry Tool
Actual riser = Total rise / Number of risers (all equal)
Used in Stair Geometry Tool
Stair Geometry Tool
Number of treads = Risers - 1
Used in Stair Geometry Tool
Blondel formula
2 x Riser + Going = 550-700 mm
Used in Stair Geometry Tool
Stair Geometry Tool
Pitch angle = arctan(Riser / Going), keep under ~42 deg
Used in Stair Geometry Tool

Wood Finish Drying Time

Open the Wood Finish Drying Time →
Wood Finish Drying Time
Temperature factor = 2 ^ ((25 - Temp) / 10)
Used in Wood Finish Drying Time
Wood Finish Drying Time
(drying roughly halves per +10C, doubles per -10C)
Used in Wood Finish Drying Time
Wood Finish Drying Time
Humidity factor = 1 + (RH - 50) / 100
Used in Wood Finish Drying Time
Wood Finish Drying Time
Adjusted time = Base time x Temp factor x Humidity factor
Used in Wood Finish Drying Time
Wood Finish Drying Time
Project time = (Coats - 1) x Recoat + Full cure
Used in Wood Finish Drying Time

Common questions

Where do these formulas come from?

Every formula on this page comes from a published source — standards, handbooks or long-established trade rules for construction and structural work. The source is named next to each formula. Nothing here is invented by us.

Can I use these formulas in my own spreadsheets or software?

Yes. The formulas themselves are public knowledge and free to use anywhere. If our page helped you, a link back is appreciated but not required.

Are the calculators on this site using these exact formulas?

Yes. Each calculator runs the formula shown here, and automated tests check the results against worked examples from the original sources before anything goes live.