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If you are wondering where to find extension spring calculators look no further. Planetspring.com offers an extension spring calculator that was constructed with you in mind. We have made the simplest and most effective extension spring calculator on the web today. You can calculate a design for small extension springs, 24 inch extension springs, or large extension springs. This calculator will help you from A-Z with your extension spring design. Before you get started on calculating your extension springs you will need to know how to measure an extension spring. Please see the picture at the top of the calculator or watch our “how to measure an extension spring” video to learn how to measure the wire diameter, free length, outer diameter, and you will need to know what type of hooks your extension springs have because these will be the 4 inputs needed to begin calculating spring rate. Extension springs are often metal springs. In order to calculate your spring properly you will need to know the type of spring wire. For example, are you looking for music wire springs or stainless steel small springs? Simply choose the type of wire and you are ready to calculate. Once you have hit the calculate button you have created your extension spring design. You have outputs such as the spring rate and the spring constant as well as any other output you will need. If you need any additional help using the calculator, or if you want to be walked through the process step by step please watch our "extension spring calculator video at the top of the page.

If you are a registered buyer with Planetspring.com our extension spring calculator offers an additional benefit. Once you have hit the calculate button you will be given 18 outputs that add up to all of the dimensions a supplier needs to quote your job. Therefore, once you have the outputs you simply copy and paste them right into the part description section of your RFQ. This allows those buyers who do not have blueprints of their spring to have a complete design before posting a RFQ.

The rate of your spring is a constant. This constant will let you know how much weight (pounds, ounces, grams, N/mm) is needed to move the spring one inch (1”) or in the case of metric measurements 1mm of distance.

The load of a spring is how much force in weight (pounds, ounces, grams, N/mm) needed to move a spring a particular distance. For example you have a 10” (inch) long spring and a rate of 10lbs/in. You want to know what the load is if you need to pull the spring 3 inches (making the total length of the spring 13 inches). So you take the rate (10lb/in) and multiply it by the distance you are traveling (3 inches). This will give you a total load of 30lbs. For extension springs you need to add the initial tension to this load to get the total load.

Formula to determine load is:
(Rate x Distance Traveled) + Initial Tension= Total Load

Formula to determine Initial tension
Initial tension = Stress (wire size)3 / 2.55 (mean diameter)

Also with an extension spring you will have a preload (L1) and a working load (L2). For example on our spring above our preload (L1) would be 10lbs @ 11 inches loaded height. L2 would be 30lbs at 13 inches height if these were the heights in your design.

The next thing you want to remember when designing your spring is the index. The index of your spring will let you know the stress level of your spring. This value is one of the outputs you will get from our calculator. Once you have calculated your spring take a look at the “spring index”. If the value in that column is lower than a 4 you will have a very high stress spring with small amounts of travel. This will cause the spring to become very fatigued and you will have spring failure. If your index is a 14 or higher your spring will be very wobbly. An example of this would be a slinky. This type of spring can be made but will come at a high cost to you because it is very hard to manufacture. If you would like optimum repeatability and a cost effective spring make sure your spring index is between 6 and 13.

Formula to determine index:
Index = mean diameter / wire size
Mean diameter = Outer Diameter - 1 Wire Diameter

Configuring a spring rate calculation by hand, calculating spring constant or calculating any type of spring constant physics can be tricky. However, if you would like to use the extension spring formulas for your calculation please visit our extension spring technical article.

Extension Spring Calculator video
Watch our extension spring calculator video here
How to measure Extension Springs video
See how to measure extension springs here
Choose Your Hook Type

Machine Hooks Croos Over Center Hooks
Side Hooks/Loops No Hooks
Extended Hooks
Hook Length 1:
Hook Lenght 2:
Enter Your Spring Dimensions

Please select your desired unit of measure:   English Metric
Diameter of spring wire, d: IN MM
Outer diameter of spring, Douter: IN MM
Length Inside Hooks, Lih: IN MM
Please select a material: