Four hundred metres means 16 lengths in a 25-metre pool and eight in a 50-metre pool.
That gives you eight extra turns in the shorter pool. Each brings another push from the wall. The distance is identical, but the way you cover it changes.
A study published in December 2025 tested 11 trained male swimmers over controlled 400-metre front-crawl efforts in both pool lengths. At matched swimming speeds, the researchers estimated that energy cost was about 4% higher in the 50-metre pool.
That estimate came from group data under tightly controlled conditions. It is not a personal pace conversion, and the study does not establish the same advantage for beginners or every swimming stroke.
It does give a reason to pause before treating a slower swim in a longer pool as lost fitness.
For your own training, start by giving each pool a separate benchmark. Keep a familiar aerobic set for the 25-metre pool and build another reference for the 50-metre pool. Write down the distance, rests and intended effort before chasing the clock.
On the first session after switching, I would leave room to adjust. Start at an effort you recognise, then see where the pace lands. If yesterday's target now requires an unusually hard push, reconsider the target before turning an aerobic session into a test.
Keep the comparison specific. A continuous swim and short repeats with generous rests deserve separate entries. So do different strokes. Add the pool length to the session name so you can find the right comparison later.
MOTRA turns verified heart-rate effort into Effort Points using personal zones when suitable heart-rate data is available. That captures one part of the session's demand. It cannot tell you how cleanly you turned or establish that two pool times are equivalent.
I would use the record to answer a narrow question: did this session deliver the work I planned? Pace helps, but it belongs beside the set, the pool and your experience of the swim.
When you change pools, give the new setting its own starting pace. Build your next target from what you can actually do there.