Standard rate is 3° per second
A standard-rate, or rate-one, turn is defined as 3° of heading change per second. That is a complete 360° turn in two minutes, a 180° turn in one minute, and a 90° turn in 30 seconds — the numbers every instrument student memorizes. Half-standard rate is 1.5°/s, or a four-minute complete turn, and is sometimes used at higher true airspeeds so the bank stays modest. Time is simply the heading change divided by the rate.
If you enter a from heading and a to heading, the calculator takes the shorter way as the primary change and also reports the long way (the other way around the compass). A change of 090° to 360° is 90° left (30 seconds at standard rate) or 270° right (90 seconds).
Actual rate from bank and TAS
The turn rate at a given bank and true airspeed is ω = (g tan φ) / V, the same identity used on the turn-radius calculator. Enter both optional boxes and this page will show that rate and the time it implies for your heading change. In instrument flight the usual practice is still to fly a standard-rate turn, or a 25–30° bank, whichever is less — the bank required for standard rate increases with TAS. This page does not cap the bank for you.
Sources
FAA Airplane Flying Handbook (standard-rate turn, 3° per second, two-minute turn); FAA Pilot's Handbook of Aeronautical Knowledge; FAA Aeronautical Information Manual (standard-rate in holding and instrument procedures). For radius, load factor, and stall-speed increase in a bank, see the turn-radius calculator.
Frequently asked questions
How long is a 90° standard-rate turn?
30 seconds. A standard-rate turn is 3° per second, so 90 / 3 = 30. A 180° turn is one minute; a 360° turn is two minutes.
What is half-standard rate?
1.5° per second, or a four-minute 360° turn. It is sometimes used at higher true airspeeds so the bank angle stays smaller.
Does this include roll-in, roll-out, or wind?
No. The time is the heading change divided by a constant rate. Wind, and the time spent rolling in and out, are not modelled. Use it as a planning number, then fly the instruments.