Where the speed presets come from
The 2024 Adult Compendium describes level, firm-surface walking as slow at 2.0–2.4 mph (code 17152), moderate at 2.8–3.4 mph (17190), brisk at 3.5–3.9 mph (17200) and very brisk at 4.0–4.4 mph (17220). The calculator uses the middle of each band for the headline time and the band edges for the range.
For comparison, a meta-analysis of 23,111 healthy adults by Bohannon and Williams Andrews found mean comfortable walking speeds from 1.43 m/s (about 5.2 km/h or 3.2 mph) in men aged 40–49 down to 0.94 m/s (about 3.4 km/h or 2.1 mph) in women aged 80–99, measured over short courses.