Post-Tensioning Calculator
Calculate post-tensioning tendon counts, spacing, elongation, and strand weight for concrete slabs.
About this calculator
Elongation (in), the field measurement stressing crews use to confirm proper prestress force, depends only on Slab Length -- the formula is jacking force times tendon length divided by strand area times steel modulus, and tendon length is simply Slab Length times 12. Slab Width, Slab Thickness, and Superimposed Load never enter that calculation at all, because elongation tracks a single tendon's physical stretch along its own length, not the slab's other dimensions or its load. Force per Tendon (lbs) is a fixed constant, 28,900 lbs, for every input combination in this calculator -- it represents a standard 0.5 inch, 7-wire strand stressed to a typical lock-off force, not a value the calculator solves for.
Total Tendons responds to all four inputs together: Slab Thickness increases the cross- sectional area each foot of width must balance, Superimposed Load raises the effective prestress needed, and Slab Length and Slab Width each add more running feet of slab edge that tendons must span in their respective directions. Approx Spacing follows the reverse pattern of Slab Thickness -- a thicker slab needs more tendons per foot of width, so the spacing between them tightens rather than widens as thickness increases. This tool sizes tendon count and spacing for typical slab-on-ground and elevated PT slabs; it does not replace a stamped structural design.
Inputs
Results
Total Tendons (both dir)
50
Elongation (in)
5.45
How to Use This Calculator
- Enter the slab length and width in feet and the slab thickness in inches (typical PT slabs are 6–10 in).
- Enter the superimposed dead plus live load in psf for your occupancy.
- Read Total Tendons (both directions) split into length-direction and width-direction counts.
- Compare Approx Spacing (in o.c.) against your code's maximum tendon spacing limit (commonly 8x slab thickness or 5 ft / 60 in o.c., whichever is less, per ACI/PTI) -- this is an industry reference range to check your project against, not a range this calculator's own output typically lands in. At the default inputs, Approx Spacing comes out around 24 in o.c.; it climbs toward the mid-50s only for thin, lightly loaded slabs and drops well under 24 in for thicker or more heavily loaded ones.
- Record Elongation (in) — this is the field measurement the stressing crew uses to confirm proper prestress force.
- Note Total Strand Weight (lbs) for material ordering and structural dead load calculations.
How the result changes with Slab Thickness (in)
| Slab Thickness (in) | Total Tendons (both dir) | Elongation (in) |
|---|---|---|
| 4 | 25 | 5.45 |
| 6 | 38 | 5.45 |
| 12 | 75 | 5.45 |
| 20 | 125 | 5.45 |
What each input means
- Slab Length (ft)
- Length of the concrete slab
- Slab Width (ft)
- Width of the concrete slab
- Slab Thickness (in)
- Thickness of the slab in inches (typical PT slab: 6-10 in)
- Superimposed Load (psf)
- Total superimposed dead + live load in pounds per square foot
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersSlab Length (ft) = 60, Slab Width (ft) = 40, Slab Thickness (in) = 8, Superimposed Load (psf) = 100 = 4 input(s) provided
- Calculate Total TendonsTotal Tendons50 = 50
- Calculate ElongationElongation5.45 = 5.45
- Calculate TendonsTendons20 = 20
- Calculate TendonsTendons30 = 30
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why doesn't slab width or superimposed load affect the Elongation reading?
Elongation (in) is calculated purely from tendon length (Slab Length times 12) and three fixed material constants -- jacking force, strand area, and steel modulus. Slab Width, Slab Thickness, and Superimposed Load play no part in that formula, so only Slab Length changes what the stressing crew should expect to read in the field.
Does Force per Tendon (lbs) change with slab size?
No -- Force per Tendon (lbs) is a fixed 28,900 lbs regardless of any input, since it represents the standard lock-off force for a 0.5 inch, 7-wire strand rather than a value the calculator solves for based on your slab.
Why does a thicker slab need tighter tendon spacing?
A thicker slab has more cross-sectional area to balance the required prestress, so it needs more tendons per foot of width to deliver enough total force -- and packing more tendons into the same width means the spacing between them (Approx Spacing) gets tighter, not wider.
What increases Total Tendons the most?
Slab Thickness has the largest proportional effect at the default inputs -- a 10% change in Slab Thickness moves Total Tendons about 5x as much as the same percentage change in Superimposed Load, and more than either Slab Length or Slab Width, because thickness feeds directly into the cross-sectional area each foot of width must balance. Slab Length and Slab Width still matter -- a longer or wider slab has more running feet of edge that tendons must span in their respective directions -- they just move Total Tendons less per percentage point than Slab Thickness does.
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