What this result means
PB math is exact if the course distances match. This tool takes one distance, an old clock, and a new clock. It reports the difference in seconds (negative meaning faster), the percent of the old time, and both paces.
It does not know whether the old race was downhill or the new one was a championship. It will happily say you “improved” from a wind-aided 5K to a slower honest 5K if the clocks say so — check the courses yourself.
20:00 to 19:30 on 5K is 30 seconds, 2.5%, and 6 seconds per kilometre faster. 4:00:00 to 3:50:00 on a marathon is 10:00 and 4.2%. Going from 20:00 to 21:00 is a positive change (slower); the card still shows the delta.
Do not compare a 5.0 km parkrun to a 5.2 km GPS file and call it a PB. Enter the same nominal distance.
Calculation method
Delta seconds = old − new. Percent = delta / old × 100. Paces are time ÷ distance. Displayed times round to the nearest second.
Exact arithmetic. No VDOT, no Riegel.
Illegal courses, short GPS, and different event distances make a fake PB. Age grading is a different question.
Frequently asked questions
How much did I improve from 22:00 to 20:00 in a 5K?
Two minutes, about 9.1%, and a faster per-km pace. Enter 5 km and both times.
Can I compare a 5K and a 10K?
Not here. Use compare two performances (VDOT) or the race equivalent calculator.
What if the new time is slower?
The delta is shown anyway. Improvement percent is negative.
Does this use VDOT or Riegel?
Neither. Same-distance PB math is subtraction. Different distances belong on compare two performances or the race equivalent calculator.