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Roundabout Capacity Calculator

Work out roundabout capacity instantly with clear inputs, formula shown and shareable results.

About Roundabout Capacity Calculator

Empirical roundabout models express entry capacity as a linear function of circulating flow: capacity falls by about 0.7 vehicles for each additional circulating vehicle. The intercept, capacity with no circulating traffic, rises with entry width, flare length and inscribed circle diameter, which is why widening the entry is the most effective geometric improvement.

Formulas

Linear entry capacity

Qe = intercept - 0.7 x circulating flow

Intercept

Rises with entry width, effective flare length and inscribed circle diameter

Worked example

With the default values below, Roundabout Capacity Calculator returns Entry capacity of 1,480. Change any input above and every figure here is recalculated in your browser.

Inputs used

Entry widthminimum 2 m
6 m
Circulating flow across the entryminimum 0 PCU/h
900 PCU/h
Average effective flare lengthminimum 0 m
20 m
Inscribed circle diameterminimum 10 m
40 m

Results

Entry capacity
1,480 PCU/h
Reserve capacity
100.00% %
Capacity with no circulating flow
2,110 PCU/h
Assessment
Entry operates with capacity available

Step by step

  1. 1Values used
  2. 2Linear entry capacity
  3. 3Intercept
  4. 4Entry capacity= 1,480 PCU/h
  5. 5Reserve capacity= 100.00 %
  6. 6Capacity with no circulating flow= 2,110 PCU/h
  7. 7Assessment= Entry operates with capacity available

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Roundabout capacity should be assessed with the empirical model and geometric parameters required by the road authority.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Roundabout capacity should be assessed with the empirical model and geometric parameters required by the road authority.

Frequently Asked Questions

How is the Roundabout Capacity figure worked out?

Qe = intercept - 0.7 x circulating flow. Empirical roundabout models express entry capacity as a linear function of circulating flow: capacity falls by about 0.7 vehicles for each additional circulating vehicle.

Why does entry width matter more than circle size?

Entry width determines how many vehicles can accept a gap simultaneously. A larger circle mainly increases speed, which is often undesirable, while a flared entry directly increases throughput.

What values do I need to enter?

This calculator takes 4 inputs: Entry width, Circulating flow across the entry, Average effective flare length, Inscribed circle diameter. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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