Shock Length Calculator sizing turns ride height, wheel travel and motion ratio into the collapsed and extended eye-to-eye lengths to buy, then checks a catalog shock against them.
Measure eye to eye, center of one mount to center of the other. For a stud mount, measure from the base of the stud.
Motion Ratio and Allowances
ShockWarehouse suggests taking half an inch off the collapsed length so the bump stop can compress. Add a droop allowance if limit straps stop the suspension first.
Lift or Lowering Kit
Enter the planned ride height change at the wheel. Every mount length moves by that amount times the motion ratio.
Check a Shock You Are Considering
Enter the catalog collapsed and extended lengths. Fox and King measure some mounts differently, so confirm the method on the spec sheet.
Eye-to-Eye Sizing in the Shock Length Calculator
The Shock Length Calculator works out the collapsed and extended eye-to-eye lengths a replacement shock needs so it never bottoms out before the bump stop or tops out before the suspension reaches full droop. It starts from your mount length at ride height and the wheel travel you want, then scales that travel through the motion ratio. Truck owners fitting lift kits, builders of lowered street cars, and anyone ordering universal shocks can use it to check sizes before buying.
Measuring Eye to Eye
Shock length is measured from the center of an eyelet mount or from the base of a stud mount, never from the tip of the stud. ShockWarehouse gives that same rule, and QA1 measures a stud-to-T-bar shock from the bottom of the washer under the stud. The US setting works in inches and the metric setting in millimeters, converted at 1 in = 25.4 mm.
QA1 also asks for the vehicle to sit at true ride height under its full weight before you measure. Measuring a truck with an empty bed and then carrying heavy loads can put the shock deeper into its stroke than planned. If the load changes a lot, the Axle Weight Distribution Calculator shows how much extra weight each axle carries.
Two Ways to Enter Your Suspension
The “Ride Height and Wheel Travel” mode takes the mount length at ride height plus the wheel travel you want up and down. The presets cover a street baseline of 3 in up and 2.5 in down, a 2 in minimum each way, and an off-road setup of 4 in up and 5 in down. It suits new builds and lift kits where the final travel is a target rather than a measurement.
The “Measured on the Vehicle” mode of the Shock Length Calculator takes three real measurements instead. These are the static mount length, the gap between the bump stop and its contact point, and the mount length at full droop. The Bilstein Academy describes the same method, where you lift the vehicle by the frame until the tire just leaves the ground to find the extended length.
From Wheel Travel to Collapsed and Extended Length
Each inch of wheel travel is multiplied by the motion ratio first, because on many suspensions the shock moves less than the wheel. Wheel travel up is subtracted from the ride height length, and a bump stop allowance comes off after that. Wheel travel down is added to the ride height length to find the extended length.
$$L_{collapsed} = L_{ride} – (x_{up} \times MR) – a_{bump}$$
$$L_{extended} = L_{ride} + (x_{down} \times MR) – a_{droop}$$
With 18 in at ride height, an upright solid axle at 1.00, 4 in up, and 5 in down, the shock reaches 14.00 in at full bump and 23.00 in at full droop. Taking off the 0.5 in bump stop allowance that ShockWarehouse suggests gives a shock of 13.50 in collapsed and 23.00 in extended. Summit Racing subtracts a full inch instead, which lowers the collapsed target to 13.00 in and leaves the bump stop more room to compress.
Motion Ratio and Shock Angle
QA1 puts the motion ratio of a shock mounted on a solid axle at about 1.0 and a typical independent front suspension at 0.50 to 0.66. Its guide also notes that an angled shock loses some movement, and Off-Road.com points out that a leaning shock travels less than a vertical one. The layout menu reflects this with an upright solid axle preset at 1.00 and an angled one at 0.90.
Switching the default truck to the angled preset changes the shock to buy to 13.90 in collapsed and 22.50 in extended, with 8.60 in of stroke instead of 9.50 in. That 0.90 in difference is enough to pick the wrong part from a catalog. Choose the preset that matches how your shocks actually sit, or measure the ratio if the mounts are unusual.
Lift and Lowering Kits
In the Shock Length Calculator, a lift or drop changes every mount length by the height change at the wheel times the motion ratio. A 4 in lift on the default upright solid axle adds the full 4.00 in, so the shock to buy becomes 17.50 in collapsed and 27.00 in extended. The stroke stays at 9.50 in, because the kit moves the whole travel window without making it longer.
ShockWarehouse suggests asking the kit maker for its recommended shock sizes first, since kits often change bump stops and droop limits too. The tool is most useful when that information is missing or when the kit has been combined with other parts. A drop works the same way in reverse, shortening both lengths by the drop times the ratio.
What the Four Result Cards Mean
The hero figure gives the collapsed and extended lengths to shop for, with the metric or US equivalent underneath. Buy a shock whose collapsed length is no longer than the first figure and whose extended length is close to the second. The four cards below it check the stroke, the ride height position, the street travel minimum, and any shock you are considering.
Stroke and Ride Height Position
The first card shows the stroke needed, which is 9.50 in or 241 mm at the defaults, along with the mount length at full bump. The second card shows how far into the stroke the shock sits at ride height. The default sits 47% in, with 4.50 in of compression room and 5.00 in of rebound room.
QA1 recommends setting ride height near the middle of the stroke, slightly biased toward compression for street use. A reading well above or below 50% means the shock will run out of travel sooner in one direction. Moving the ride height or choosing a different shock length brings it back toward the middle.
Street Minimum Travel Check
QA1 advises aiming for at least 2 in of compression and 2 in of rebound, with a street baseline of about 2.5 to 3.0 in up and 2.0 to 2.5 in down at the wheel. The third card converts the stroke on each side back to wheel travel and compares it with 2 in. The defaults clear it by 2.50 in on compression and 3.00 in on rebound.
Checking a Catalog Shock
The last panel of the Shock Length Calculator takes the collapsed and extended lengths of a shock from a catalog and compares them with your mounts. The default 13.25 in by 23.25 in shock leaves 0.75 in before the bump stop and reaches 0.25 in past full droop, so it fits. A shock whose collapsed length is too long would bottom out before the bump stop and take the hit through the shock body.
If the checked shock tops out before full droop, the alert warns that it would act as the droop limiter. QA1 says bump stops and limit straps, not the shock, should limit the travel, so add straps or choose a longer extended length. Once the length is settled, the Shock Force Calculator helps pick the damping for the same corner.
Shock Length Calculator Limits
Lengths and the motion ratio must be above zero, and travel and allowances cannot be negative. In measured mode the full droop length must be longer than the static length, and a checked shock must extend longer than it collapses. If the allowances leave no room between the two lengths, the tool reports that no shock can fit those numbers.
The alert also flags a stroke longer than the collapsed length, since the shock body has to be long enough to hold the shaft. The math treats shock movement as a straight fraction of wheel movement, while real motion ratios change a little through the travel. Cycle the suspension slowly after fitting new shocks and check the driveshaft, brake lines, and sway bar links at full droop.
Measuring Mistakes That Change the Size
Measuring a stud mount from the tip instead of the base adds the stud length to every figure and makes the shock look longer than the space allows. ShockWarehouse and Summit Racing both measure a stud from its seating surface for that reason. Use the same point on both ends every time you measure.
Measuring ride height with the vehicle on jack stands, or with the bed empty when it usually carries weight, puts the shock in the wrong part of its stroke. Off-Road.com also suggests disconnecting the sway bar when measuring droop, because the bar can stop the axle short of its real travel.
Using wheel travel directly as shock travel on an independent suspension overstates the stroke needed by half or more, since QA1 puts those ratios at 0.50 to 0.66.
Shock Sizing Questions From Owners
How do I know what length shocks I need?
Measure the mount length at ride height, the gap to the bump stop, and the mount length at full droop, then subtract a bump stop allowance from the compressed figure. The Shock Length Calculator does this for you and scales the numbers through the motion ratio. For the default upright solid axle truck, the result is a shock of about 13.50 in collapsed and 23.00 in extended.
What length shocks do I need for a 4 inch lift?
Add the lift times the motion ratio to both lengths. On the default upright solid axle at 1.00, a 4 in lift moves the target from 13.50 by 23.00 in to 17.50 by 27.00 in, while the stroke stays at 9.50 in. Check the kit maker’s recommendation first, because many kits also change the bump stops or add limit straps.
What is the difference between collapsed and extended length?
Collapsed length is the shortest eye-to-eye length the shock reaches when fully compressed, and extended length is the longest when fully open. The difference between them is the shaft travel, or stroke, as members of the Ford Truck Enthusiasts forum point out when comparing shocks. A 13.25 in by 23.25 in shock has 10 in of stroke.
What happens if a shock is too long or too short?
A shock with too long a collapsed length bottoms out before the bump stop, which can damage the shock and its mounts. A shock with too short an extended length tops out early, limits droop, and can be pulled apart over rough ground. The fit check in the calculator shows both cases before you buy.
Should the shock limit how far the suspension droops?
The shock should not be the part that stops the suspension at either end of its travel. QA1 recommends keeping a margin so bump stops and limit straps end the travel instead. Choose an extended length that reaches slightly past full droop, or add limit straps if the shock you want is shorter.