Car Turning Radius Calculator

The circle a car needs at full lock, curb-to-curb and wall-to-wall, from its wheelbase and steering angle or back from the figure in the brochure, with the band it sweeps drawn to scale.

I know the

Units

Steering

Steering angle at full lock

deg

The angle is the

The car

Wheelbase

Front track (wheel centre to centre)

Body width (no mirrors)

Tire width (optional)

Front overhang, axle to bumper

Rear overhang, axle to bumper

Wheelbase, track and width are on the spec sheet. Overhangs are front axle centre to the front bumper face and rear axle centre to the rear bumper face; they only affect the wall-to-wall figures. Leave the tire width blank for a bare centreline calculation.

At full lock, from above

Every radius from the turning centre

Point on the car	Radius

The space it takes

Measure	Value

Low-speed geometry only: the car is assumed to roll with no tire slip, every wheel tangent to its own circle about one centre on the rear axle line, which is what a walking-pace full-lock turn does. Track stands in for the kingpin spacing Ackermann strictly uses, so the wheel angles are a degree or so from what an alignment rack prints; the radii are not affected. Real cars rarely reach perfect Ackermann, and their published circles are measured on the road, so a brochure figure and this page can disagree by a few percent. Nothing here says how fast the car can take the circle: that is grip, not geometry.

Curb-to-curb, wall-to-wall, and why the brochure number is a diameter

A turning circle is the smallest circle a car can drive round at full lock, and it is quoted as a diameter even when the brochure says radius. Curb-to-curb is the circle the outside front tire traces: the minimum distance between two raised curbs, both lower than the bodywork, in which the car can make a U-turn. Wall-to-wall is the circle the outermost point of the body traces, which is the outer front bumper corner because it hangs out beyond the front axle, and is the minimum distance between two walls taller than the car. The wall figure is always larger, by about twice the reach of that corner beyond the wheel, and it is the one that decides whether a car gets round a gatepost or a garage entrance. Cars sold in the EU are type-approved on a wall-to-wall annulus of 12.5 m outer and 5.3 m inner radius, and the space table checks a car against it.

Everything turns about one point

At walking pace with no tire slip, every wheel on the car rolls along a circle, and all those circles share one centre that lies on the line of the rear axle. That single fact gives every number on this page. The rear axle centre sits at the wheelbase divided by the tangent of the average steering angle; each front wheel points at the same centre, so the inner one steers further than the outer one, which is what Ackermann geometry means; and any other point on the car, a tire edge or a bumper corner, is just a right triangle away from the centre. The page takes whichever angle you have, the outer wheel that the curb formula uses, the inner wheel an alignment rack reports, or the average a textbook calls the bicycle model, and derives the other two.

The band the car sweeps

Because every point turns on its own radius, the car does not stay inside its own width during a turn. The outer front corner runs on the largest circle and the inner rear wheel on the smallest, so the road the car actually covers is the ring between them, wider than the car by the whole difference. For an ordinary car that band is about one and a half to two car widths, and it is the number a driveway, a gate or a parking aisle has to allow, not the car's width and not the turning circle. The rear wheels cut inside the path of the front wheels by the off-tracking figure in the table, which is why a tight corner clips the inside curb with a rear tire long before the front gets close. The drawing shows the ring to scale with the car placed in it.

Working back from the published figure

Manufacturers publish the turning circle and almost never the steering angle behind it. Given the wheelbase and track, the circle fixes the geometry completely, so the second mode takes the brochure figure and returns the angles the front wheels must reach at full lock. That is the number to compare when a lift kit, a wider tire or a different offset starts rubbing at full lock, and it is a quick sanity check on the brochure itself: a passenger car whose outer wheel comes out below about 25 degrees, or whose inner wheel comes out above about 45, has a wheelbase or a radius-for-diameter mix-up somewhere. The tire size calculator gives the overall tire diameter and the wheel offset calculator the clearance at the strut, the two things that change what full lock is allowed to be.

What this tool will not do

It will not tell you how fast the car can go round the circle, because that is tire grip and weight transfer rather than geometry, nor what happens on a trailer, a four-wheel-steer car or a truck with a steered rear axle, which turn about different centres. It uses the track where Ackermann strictly wants the kingpin spacing, so the wheel angles are an approximation of a degree or so even though the radii are exact for the dimensions typed. And it treats the car as a rectangle: a rounded bumper reaches a little less far than the corner the wall-to-wall figure assumes, which is the safe side for clearance. For the angles the alignment rack sets rather than the ones full lock reaches, the toe and camber calculator is the neighbour to use.

Choose what you know