Hours, minutes, and tenths
Exact decimal hours are just hours plus minutes divided by 60: 1 hour 30 minutes is 1.5, and 1 hour 20 minutes plus 2 hours 45 minutes is 4 hours 5 minutes (4:05, or 4.083… hours). Many U.S. logbooks are kept in tenths of an hour, with 6 minutes equal to 0.1 hour. The usual mapping, implemented here with ordinary half-up rounding of minutes to the nearest multiple of 6, is:
0–2 min → 0.0 · 3–8 → 0.1 · 9–14 → 0.2 · 15–20 → 0.3
21–26 → 0.4 · 27–32 → 0.5 · 33–38 → 0.6 · 39–44 → 0.7
45–50 → 0.8 · 51–56 → 0.9 · 57–60 → 1.0
So 1:03 logs as 1.1 in tenths, and 1:02 stays 1.0. That convention is a widespread practice, not something 14 CFR prescribes. You can type a duration as 1:30 or as a decimal such as 1.5.
Categories and period windows
14 CFR 61.51 is the rule that says what a pilot logbook must record and what time may be logged as PIC, SIC, training, instrument, night, and cross-country, among other items. This page does not decide whether a given flight may be logged in a category — it only adds the time you mark. One flight can belong to several categories at once (for example a night cross-country as PIC counts as total, PIC, night, and cross-country).
When entries have dates, the calculator also reports totals in the preceding 90 days, a rolling 6 months, a rolling 12 months, and the calendar year through the “as of” date. Those are bookkeeping windows. They are not the 61.57 passenger-landing or instrument-experience tests, which depend on landings and tasks rather than on a time total. For those rules, use the pilot currency checker.
Sources
14 CFR 61.51, Pilot logbooks; FAA Pilot's Handbook of Aeronautical Knowledge (logbooks and categories of time). The tenths table is the common U.S. 6-minute = 0.1 hour practice, not a regulatory rounding rule.
Frequently asked questions
How do I convert 1 hour 3 minutes to logbook tenths?
1.1 hours. The usual U.S. practice rounds to the nearest tenth of an hour (6 minutes). 3 minutes rounds up to 6 minutes, so 1:03 is logged as 1.1. 1:02 stays 1.0.
Does a night cross-country as PIC get added more than once to total time?
No. Each entry contributes its duration to total time once. Checking PIC, night, and cross-country adds that same duration to each of those category totals, not to the overall total again.
Do the 90-day totals tell me whether I am current to carry passengers?
No. 61.57 passenger currency is about landings (and, at night, full-stop landings) in the preceding 90 days, not about a time total. Use the pilot currency checker for those rules.