# String Tension AI > String Tension AI is an iOS app that measures tennis racquet string tension with your iPhone's microphone - no extra hardware. Tap the stringbed and get a tension reading in lbs (and kg) in about 30 seconds, track tension loss over time, and know when to restring. String Tension AI turns an iPhone into a string-tension tracker for tennis. Players tap their strings, get a tension estimate in seconds, track how tension fades with play, and see when it may be time to restring. The app is free to start. This site is the product's marketing and information hub. ## Key facts - Platform: iOS app for iPhone (requires iOS 17 or later). Free to download and free to measure; an optional PRO upgrade adds advanced analytics and recommendations. - App Store rating: 5.0 out of 5 (130 ratings). - What it measures: tennis racquet string tension, reported in lbs (primary unit) and kg. - How it works: tap the stringbed and the iPhone microphone analyzes the resulting sound. No clip-on sensors, Bluetooth gadgets, or extra hardware are needed. - Speed: about 30 seconds from opening the app to a reading, using 3-10 guided taps. - Tracking: saves a fresh-string baseline per racquet, follows tension loss over time, and signals when to restring. - Privacy: stringbed audio is processed on-device. - Publisher: Trader Prosperity LLC. ## Main pages - [Home](https://stringtensionai.com/): Product overview - what the app does, key features, how it works, the science, FAQ, and reviews. - [Tennis String Resources](https://stringtensionai.com/resources/): The complete library of measurement guides, tension references, string-life research, professional setup data, and private browser tools. - [About the Research Team](https://stringtensionai.com/about/): Editorial experience, evidence standards, public-data methodology, update policy, and corrections. - [Tennis String Tension App](https://stringtensionai.com/tennis-string-tension-app/): How the iPhone app measures tension, tracks tension loss over time, and signals when to restring. - [Measure Tennis String Tension](https://stringtensionai.com/measure-tennis-string-tension/): Guide to measuring string tension with an iPhone (no hardware needed), with a free in-page tension calculator. - [Racquet String Tension Checker](https://stringtensionai.com/racquet-string-tension-checker/): A private browser tool that compares a current reading with a fresh-string baseline and preferred floor in lbs, kg, and percent. - [When to Restring Your Tennis Racquet](https://stringtensionai.com/when-to-restring-tennis-racquet/): When to restring by string type and playing hours, with a free restring calculator, a tension-loss chart, and FAQ. ## Editorial resources - [2026 Pro String Setups](https://stringtensionai.com/pro-string-setups/): A filterable, source-linked catalog of 69 reported professional string setups with last-verified dates and matched racquet context where available. - [Tennis String Tension Chart](https://stringtensionai.com/tennis-string-tension-chart/): Frame-range-first starting tensions organized by string construction and player objective, in lbs with kg secondary. - [Best Tennis String Tension for Control](https://stringtensionai.com/best-tennis-string-tension-for-control/): Source-backed control-oriented starting ranges with an interactive frame-and-string overlap calculator. - [Best Tennis String Tension for Spin](https://stringtensionai.com/best-tennis-string-tension-for-spin/): An evidence-first spin guide with an interactive starting-point tool, lower-versus-higher tension comparison, controlled testing method, and source-linked FAQ. - [Tennis String Tension Loss](https://stringtensionai.com/tennis-string-tension-loss/): An interactive calculator and guide comparing directional tension-loss curves by string construction, playing frequency, and time since stringing. - [Poly String Tension Loss](https://stringtensionai.com/poly-string-tension-loss/): A source-backed timeline explaining early relaxation, playing-hour change, measurement, and replacement signals. - [How Long Do Poly Tennis Strings Last?](https://stringtensionai.com/how-long-do-poly-strings-last/): A source-backed guide to finding a personal polyester restring window from playing hours, tension history, and repeatable performance signals. - [Tennis String Lifespan by Hours](https://stringtensionai.com/tennis-string-lifespan-by-hours/): A broad comparison of polyester, multifilament, synthetic gut, natural gut, and hybrid lifespan signals, with a framework for tracking personal useful life by playing hours. - [Poly vs. Gut String Tension Loss](https://stringtensionai.com/poly-vs-gut-string-tension-loss/): A source-backed material comparison with a measured-retention calculator, gut/poly hybrid guidance, fair-test protocol, and FAQ. - [Dead Tennis String Symptoms](https://stringtensionai.com/dead-tennis-strings/): A six-symptom matrix and three-signal test for deciding when a stringbed has left a player's usable range. - [String Cost-Per-Hour Calculator](https://stringtensionai.com/string-cost-per-hour-calculator/): A private browser tool for comparing two setups by installed cost, playable hours, restring cadence, and annual spend. - [Tennis String Cost Tracker](https://stringtensionai.com/tennis-string-cost-tracker/): A private browser planner for estimating one setup's cost per playing hour, restring cadence, job equivalents, and a 3- to 24-month budget from visitor-entered values. ## App download - [String Tension AI on the App Store](https://apps.apple.com/app/apple-store/id6763697805?pt=118521925&ct=llms_txt&mt=8): Download the iOS app — free to start, rated 5.0/5 from 130 ratings. ## Machine-readable public data - [Pro string setups 2026 JSON](https://stringtensionai.com/docs/pro_strings_2026.json): The 69-record distribution behind the filterable pro setup page, including mains, crosses, gauges, tensions, tours, verification dates, and source classifications. - [Pro racquets 2026 JSON](https://stringtensionai.com/docs/pro_racquets_2026.json): Professional racquet records with brand, model, head size, weight, pattern, and related catalog fields. - [Pro equipment editorial 2026 JSON](https://stringtensionai.com/docs/pro_equipment_editorial_2026.json): Editorial evidence and source metadata for professional equipment records. - [Tension guidance 2026 JSON](https://stringtensionai.com/docs/tension_guidance_2026.json): Public reference guidance for string constructions and tension decisions. The four files above are intentionally allowed in robots.txt. Other files under `/docs/` remain excluded because they are local notes or non-public reference material. ## Optional - [Privacy Policy](https://stringtensionai.com/privacy/): How app and website data are handled, including on-device audio processing. - [Terms of Service](https://stringtensionai.com/terms/): Terms governing use of the iOS app.