Abstract This paper outlines the R&D methodology, accelerated testing and successful results of an innovative Smart Reamer Calliper for measurement while drilling. The concept was progressed from design to prototype with long hours downhole drilling qualification tests completed in 3 years. Limitations often exist with ultrasonic calliper measurement technology due to bottom-hole assembly (BHA) placement and changes in fluid density. The Smart Calliper prototype tool overcame these limitations by building onboard proper calibration and multiple sensors that allow 360 ° mapping of the wellbore shape. Further optional Measurement While Drilling (MWD), vibration, magnetic and underreamer position sensors can be added. An R&D project was established with incremental tests identifying root-cause failure/design issues before moving to the next stage. This iterative process ensured proper prototype manufacturing and minimized risk at critical test points. At the same time parallel activities enabled the project to reduce overall time associated with common project tasks. The whole R&D consisted of four stages: design and finite element analysis (FEA), small scale testing, full scale testing and long hours of field test drilling. The prototype tool was first tested in sliding, rotating and underreaming BHAs for drilling a total of 900 m in chalk, clay and sands. Small scale lab tests — including high pressure and temperature at 13,053 psi and 150°C for 100 hours — were conducted in England. Full scale under-reaming testing occurred in Norway with a wireline calliper for cross-check of the wellbore diameter measurement, and finally another qualification test in England comprising 59 hours on bottom drilling. Last, a real-time data display was performed via an integration with a standard mud pulse telemetry. In conclusion, the new Smart Calliper tool accurately measures and can provide both memory and real-time data of the 3D wellbore, allowing the drilling team at the rig-site to: Pinpoint narrow or under-gauge wellbore sections, verifying underreamer performance and measuring wellbore diameter above the reamer, allowing for a corrective reaming trip to be performed while drilling BHA is still in hole.Offer better cement volume estimate to minimize excess.Eliminate wireline calliper deployment.Eliminate a wiper trip typically carried out prior to running long casing or liner string.Offer monitoring of downhole tool vibration and separate directional survey if required.