Abstract This paper presents a theory and formulas for correction of the drillstring length as a function of loads, such as tension force, fluid pressures and temperature. The effects being specifically addressed are static and dynamic tensional stretch, pressure ballooning, hydrostatic shortening, flow lift shortening, and thermal expansion. Many of these corrections increase progressively with the drillstring length and the sum of all correction can reach several meters for long drillstrings. The corrected, load dependent drillstring length is the key to estimate both the measured well depth and the short-term rate of penetration (ROP) more accurately. Accurate short-term ROP is important both for calculating mechanical specific energy (MSE) and for monitoring and optimization of the drilling process.