Calculate turnaround time (TAT) as completion minus arrival time, solve for arrival or completion time, or find FCFS average turnaround and waiting time.
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Turnaround Time Formula
Turnaround time is the total time from when a process arrives until it is completed.
- TAT = turnaround time
- CT = completion time, the time when the process finishes
- AT = arrival time, the time when the process enters the ready queue or system
For a single process, the calculator subtracts arrival time from completion time. All entered times and results use the selected unit. Changing the unit reinterprets the numbers; it does not convert them.
For this simplified FCFS schedule, one CPU handles each process in arrival order without preemption, I/O waits or context-switch overhead. Equal arrivals retain entered order. If the CPU is idle before the next process arrives, the current time moves forward to that process’s arrival time.
- CTi = completion time for process i
- ATi = arrival time for process i
- BTi = burst time for process i
- TATi = turnaround time for process i
- n = total number of processes
The FCFS mode calculates each process completion time, finds each turnaround time, adds all turnaround times, then divides by the number of processes to return the average turnaround time.
Common CPU Scheduling Time Terms
| Term | Meaning | Used in this calculator? |
|---|---|---|
| Arrival Time | The time when a process becomes available for execution. | Yes |
| Burst Time | The amount of CPU time a process needs. | Yes, in FCFS mode |
| Completion Time | The time when a process finishes execution. | Yes |
| Turnaround Time | The total time from arrival to completion. | Yes |
| Waiting Time | The time a process spends waiting in the ready queue. | Yes, in FCFS mode |
FCFS Scheduling Result Guide
| Result | What it means |
|---|---|
| Lower turnaround time | Processes are finishing closer to their arrival times. |
| Higher turnaround time | Processes are spending more total time in the system before completion. |
| Average turnaround time | The mean turnaround time across all processes in the input list. |
| Idle CPU gap | If the next process has not arrived yet, FCFS waits until that arrival time before starting it. |
Example Turnaround Time Calculations
Example 1: Single process
A process arrives at time 4 and completes at time 15.
The turnaround time is 11 time units.
Example 2: FCFS average turnaround time
Use arrival times 0, 3, 6 and burst times 5, 9, 6.
- P1: CT = 0 + 5 = 5, so TAT = 5 – 0 = 5
- P2: CT = 5 + 9 = 14, so TAT = 14 – 3 = 11
- P3: CT = 14 + 6 = 20, so TAT = 20 – 6 = 14
The average turnaround time is 10 time units.
Turnaround Time Calculator FAQ
What is the difference between turnaround time and waiting time?
Turnaround time is the full time from arrival to completion. Waiting time is only the time spent waiting before or between CPU execution. In simple non-preemptive scheduling, waiting time can be found with:
Can turnaround time be zero?
Yes. Turnaround time is zero if the completion time equals the arrival time. This is uncommon in CPU scheduling problems because a process usually needs a positive burst time, but it is valid for a basic time difference calculation.
Why does FCFS order matter?
FCFS means first come, first served. Processes are executed by arrival time, not by shortest burst time or priority. A long process near the front can increase the turnaround time of later processes because they must wait for it to finish.
