Read equipment data with confidence
Tennis String Stiffness Explained
Short answer: Stiffness describes resistance to deformation under a specified test. It is different from the tension you request on the stringing machine. A single-string laboratory stiffness result, a whole-stringbed reading and a frame RA rating describe different things. Compare like-for-like measurements to choose a candidate, then test the complete setup on court; a lower number alone does not prove better comfort, more spin or a better match for you.
Keep the quantity with the number
Four measurements that should not share one scale
| Number | What it describes | What it does not establish |
|---|---|---|
| Reference tension | The requested stringing-machine force, such as 50 lbs (22.7 kg). | The finished bed’s current reading, every individual string’s tension or its dynamic stiffness. |
| Single-string laboratory stiffness | Resistance to deformation in a defined test; TWU reports string stiffness in lb/in. | The full racquet’s feel, a universal tension setting or an injury-risk score. |
| Whole-stringbed stiffness / DT | The response of the installed bed. ERT defines DT in terms of force per central deflection. | A direct reading of the stringing-machine setting or equivalence to a single-string laboratory number. |
| Frame RA | An index from a static frame-deflection test, described in HEAD’s racquet guidance. | String stiffness, the full pattern of frame flex or your comfort with the complete racquet. |
Keep the method and unit next to every result. A DT of 35 is not 35 lbs. A laboratory value of 200 lb/in is not a request to string a racquet at 200 lbs. For other terms, use the stringing glossary.
ERT’s own explanation describes its tension conversion as approximate. HEAD’s RA explanation also notes that one static measurement does not describe every aspect of dynamic frame behavior. Neither should be reduced to a universal player score.
The test speed matters
Static stretch and impact response are different questions
A string that stretches readily during a slow pull need not respond the same way during a quick impact. TWU’s measurement explanation distinguishes static and dynamic stiffness and shows why the initial test tension belongs with the result.
For a first comparison, ask whether both values came from the same method at the same selected conditions. Mixing two tables without that check can create a false ranking. A specification with no method is less useful than a slightly less convenient source that explains how it obtained the number.
Lab stiffness is one input to a shortlist. When you compare the complete racquet, installation, geometry, wear and the rest of the setup are also present. A phone tension estimate is not a new laboratory test of the individual string.
A numerical example
What a percentage difference actually tells you
These are fictional strings and invented values, not a product comparison. Imagine that String A measures 180 lb/in and String B measures 210 lb/in under the same laboratory conditions.
B’s measured stiffness is about 16.7% higher: the 30 lb/in difference divided by A’s 180 lb/in. That is a comparison of the measured property under those conditions. It is not evidence of 16.7% more control, less comfort or any particular change in your results.
If A and B were tested at different tensions or by different procedures, even that simple ranking needs qualification. Do not translate the 30 lb/in gap into a 30 lbs stringing adjustment; force per distance and force are different quantities.
A checklist before using a lab table
- Match the identity: product name, version and diameter, rather than the family name alone.
- Match the test: selected tension, procedure and impact condition where the source provides them.
- Read the conditioning: whether the string was fresh, settled or tested after impacts.
- Keep other properties separate: tension loss and friction-related results answer additional questions.
- Record uncertainty: an unexplained small difference is not proof you will notice it.
The TWU comparison tool presents several properties separately. Our string database provides manufacturer specifications such as construction and gauge; those specifications are not laboratory stiffness measurements.
Use labels as questions
What “soft poly” can and cannot tell you
A product label can tell you how a maker positions a string. It does not provide a common measurement scale across brands and materials. “Softer than another poly in our range” is a narrower claim than “equivalent to this multifilament in your racquet.”
Look for evidence about the exact candidate. If comparable stiffness data is unavailable, write down the intended difference and test it rather than assigning a numerical ranking from the packaging. The polyester decision guide helps decide whether poly is the right category to test at all.
Similarly, a lower later-life tension reading does not mean the entire setup has become better for you. Use wear, response and the job’s history alongside readings; the dead-string guide explains why physical survival and useful play are separate endpoints.
Turn data into a decision
Choose one candidate, then test the complete setup
Start with a concrete need, such as easier defensive depth or a response you prefer on returns. Use comparable lab information to choose a candidate in an appropriate construction and gauge. Agree on an installation inside the frame and string guidance; do not try to calculate a universal tension conversion from stiffness scores.
Keep a record of the string job and a consistent measurement routine. Temperature belongs in that routine: Lindsey and Cross’s original experiments demonstrate that test conditions can change string tension and stiffness. Compare readings under similar conditions rather than interpreting every fluctuation as wear.
Then use the on-court scorecard with familiar tasks, both early and later in the job. If you are choosing a device, the app-versus-meter guide explains the different measurements. The useful outcome is a repeatable setup decision, not the lowest number on a spreadsheet.
Quick answers
Tennis String Stiffness Explained: FAQ
Is string stiffness the same as string tension?
No. Reference tension is the force setting requested during stringing. Stiffness describes resistance to deformation under a stated test. The material, construction and test conditions matter, so identical reference tensions do not establish identical stiffness or response.
Is a lower laboratory stiffness value always better?
No. It can help identify a different response when the measurements are comparable, but it is not a complete rating of comfort, control, spin or suitability. Use it to choose a candidate and test that candidate in your racquet.
Can I compare a DT reading directly with pounds on the stringing machine?
No. ERT’s DT describes whole-stringbed stiffness, while the machine setting is reference tension. ERT itself describes its conversion to tension as approximate. Keep units and methods with each number, and compare a racquet’s later readings with its own baseline.
Does a soft-poly label mean the string is as soft as a multifilament?
No. The label may describe a relative position within a polyester range or a manufacturer’s intended feel. Look for comparable measurements for the exact product and gauge, and judge the complete setup. The label is not a standardized cross-material rating.
Sources and method
What the evidence supports
The sources define their own measurements and methods. We use them to distinguish quantities and show how to read a comparison, not to endorse every performance or accuracy claim on a manufacturer page. The worked numbers are invented for arithmetic, and the shortlist workflow is an editorial method rather than a validated predictor of player preference.
- Crawford Lindsey — String Stiffness: The Alpha and Omega of String Performance — dynamic versus static string stiffness and the importance of test tension.
- Tennis Warehouse University — Compare Strings — separate stiffness, tension-loss, energy-return and friction-related measurements; consult the selected test conditions.
- ERT Technic — Dynamic Tension Value — the manufacturer’s definition of whole-stringbed DT and its distinction from reference tension; not independent validation of device accuracy.
- HEAD — How to Choose a Tennis Racquet, frame stiffness section — RA as a static frame measurement and the limits of that single index.
- Lindsey and Cross — The Effect of Temperature on Tennis String Tension and Stiffness — original temperature experiments, relevant when controlling measurement conditions.
Sources checked October 27, 2026. See our editorial standards. A source access date is not a new experiment or an independent product test.
Track change without mixing unlike numbers
Keep a consistent method and a baseline for each string job, then connect changes in readings with the response you notice on court.
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