First In Last Out Times: How FIFO Works

This page explains what “first in, last out” (FILO) times mean in practice, whether you’re asking about inventory rotation, shift scheduling, or data structures. In short, first in last out describes a system where the item (or person) that arrives first is the last one to leave, and the most recent arrival leaves first. Below we break down how FILO works in different contexts, how to calculate the relevant times, and common misconceptions.

Formula

The core formula for a first in last out time is simply:

Time Out = Time In + Duration

Where:

• Time In (t): the moment the item or person enters the system, measured in minutes after midnight (e.g., 8:00 AM = 480).
• Duration (d): how long the item or person stays, in minutes.
• Time Out: the moment they leave, expressed as a clock time (e.g., 9:00 AM) or minutes after midnight.

This formula is the same whether you’re dealing with a stack of plates, a queue of tasks, or a shift schedule. The “first in” part tells you who has been waiting the longest; the “last out” part tells you that they will be the last to exit under a strict LIFO (last in, first out) or FILO policy.

Worked Example

Using the defaults from the calculator above:

1. Time in = 480 minutes (8:00 AM)
2. Duration = 60 minutes
3. Time out = 480 + 60 = 540 minutes
4. 540 minutes after midnight = 9:00 AM

So if the first person enters at 8:00 AM and stays for 60 minutes, they leave at 9:00 AM. In a FILO system, they would be the last to leave among all who entered after them—but their individual exit time is still just entry plus duration.

Where FILO Times Appear

First in last out is a concept used in several unrelated fields. Understanding which one you’re dealing with is key to interpreting the times correctly.

Inventory and Stock Rotation

In warehouses, FILO (also called LIFO, last in first out) means the newest stock is sold or used first, leaving the oldest stock to sit until last. This is the opposite of FIFO (first in first out), where the oldest stock leaves first. The “time” here is the shelf life or storage duration. For example, if you receive 100 units on Monday and 100 on Tuesday, under FILO the Tuesday units are shipped out first, so the Monday units might stay for weeks. Calculating the actual “last out” time requires knowing the depletion rate, not just the arrival time.

Shift Scheduling and Employee Time

In workplaces, “first in last out” often refers to seniority: the employee with the most seniority (first in) gets to leave last on busy days, or conversely, the last hired leaves first. The times are the clock-in and clock-out times. If a senior employee clocks in at 8:00 AM and works a 9-hour day, they clock out at 5:00 PM. The “last out” might mean they stay until all others have left, so their actual clock-out could be later than 5:00 PM if they cover for others.

Data Structures (Stacks)

In computer science, a stack is a classic FILO structure. The “time” is the order of push and pop operations. If you push item A at 1:00, then B at 1:01, then C at 1:02, the pop order is C, B, A. The “last out” time for A is when the stack is empty, which depends on how many pops happen. This is not a clock time but a sequence position.

How to Choose Between FIFO and FILO

Choosing the right system depends on your goal:

• Use FIFO for perishable goods, where the oldest items spoil first.
• Use FILO for non-perishable items where you want to reduce handling of older stock (e.g., coal piles) or for data structures where you need to reverse order.
• For people, use FILO (seniority) when you want to reward long-term employees with stability, but be aware it can cause morale issues.

If you’re calculating times, the formula is the same: entry time + duration. The difference is in which item you’re calculating for.

Common Mistakes and Misconceptions

A frequent error is assuming FILO means the first item leaves at the same time as the last item enters. That is only true if durations are zero. Another mistake is mixing up clock time and duration. For example, if someone works from 8:00 AM to 5:00 PM, the duration is 9 hours, but the “last out” time is 5:00 PM, not 9:00 PM. Also, in inventory, people often confuse the arrival date with the departure date—the departure date is arrival plus storage time, which is not always known in advance.

Limits of the Simple Formula

The simple formula assumes a constant duration and no interruptions. In real life, a worker might take breaks, a product might be held longer due to demand, or a stack might have items added and removed in a non-linear order. For accurate planning, you need to account for variability. The calculator above gives a baseline; adjust for real-world conditions.

Frequently Asked Questions

What does “first in last out” mean in simple terms?

It means the first thing to arrive is the last thing to leave. Imagine a stack of plates: the plate put on the bottom is the last one you pick up.

How do I calculate last out time for a shift?

Take the clock-in time and add the shift duration. For example, clock in at 9:00 AM, work 8 hours, clock out at 5:00 PM. If you also need to cover for others, add that extra time.

Is FILO the same as LIFO?

Yes, in most contexts FILO and LIFO are synonyms. Both mean last in, first out. The term FILO emphasizes the first item’s position, while LIFO emphasizes the last item’s priority.

Why would a company use FILO instead of FIFO?

Companies use FILO when the oldest items don’t spoil or become obsolete, and when moving older stock is costly. For example, in a coal pile, the oldest coal is at the bottom and is left until last because it’s expensive to dig it out.

Does FILO apply to people in a queue?

No, a queue is typically FIFO—the first person in line is served first. FILO would mean the last person in line is served first, which is unusual for people but common for data stacks.

Can I use the calculator for inventory storage times?

Yes, if you know the arrival time and the duration the item will be stored. Enter the arrival time as the “time in” and the storage duration as the “duration.” The result is the expected departure time.

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in t | Time in (minutes after midnight) | min | 480 | 0
in d | Duration (minutes) | min | 60 | 0
out out_t | Time out (clock) | clock | t + d | 0
out out_dur | Time out (minutes) | min | t + d | 0