Shelf Life Calculator

Q: What is the difference between best by, use by, and sell by dates?

They mean different things: Best by (or best before): a quality date. After it, food may lose flavor or texture but is often still safe. Use by: the last day the maker says the product is at its best. On baby formula and some fresh foods, treat it as a safety limit. Sell by: a message to the store, not to you. It tells staff when to pull the item. Food usually stays good for a while after it. Only infant formula is federally date-regulated in the United States. Most other dates are set by the maker.

Q: What is the difference between FIFO and FEFO?

FIFO (First In, First Out) moves the oldest delivery first. FEFO (First Expired, First Out) moves the item with the closest expiration date first. They are not always the same. A box that arrived last month may expire after a box that arrived today, if the newer box was made earlier. For dated goods like food and medicine, FEFO is safer because it follows the real expiration date, not the delivery date.

Introduction

The Shelf Life Calculator tells you how much time a product has left before it expires. You enter three dates: the date it was made, today's date, and the expiration date. The calculator then shows the total shelf life, the days that have passed, and the days remaining.

It also shows what percent of the shelf life is used and what percent is left. A status badge tells you if the product is Good, Use Soon, Near Expiry, or Expired. A timeline chart and a step-by-step solution show you how the math works.

You also get three milestone dates: when 75%, 50%, and 10% of the shelf life is left. These help you plan when to sell, use, or reorder a product before it goes bad.

This tool works for food, medicine, cosmetics, chemicals, and any other product with an expiration date. Use it to check stock, rotate inventory, or just see if something in your kitchen is still good.

How to use our Shelf Life Calculator

Enter three dates for your product, and the shelf life calculator shows how many days are left, how much shelf life you have used, the product status, and key milestone dates.

Date of Manufacture: Pick the day the product was made or packed. This is the start of its shelf life.

Today's Date: This fills in with today by default. Change it to any date if you want to check a "what if" case, like a future ship date.

Expiration Date: Pick the date printed on the label. It must come after the manufacture date.

Duration Display Unit: Choose how you want the time shown. "Auto" picks the best fit, or you can pick days, weeks, months, or years.

Click Calculate to see your results, the step-by-step math, and the timeline chart. Click Reset to clear the dates and start again.

What Is Shelf Life?

Shelf life is how long a product stays safe and good to use. It starts on the day the product is made and ends on its expiration date. Food, medicine, makeup, paint, and chemicals all have a shelf life.

The Two Dates That Matter

Date of manufacture: the day the product was made, packed, or bottled. Some labels call this the production date or "MFG."
Expiration date: the last day the maker says the product is at full quality. You may also see "best by," "use by," or "EXP."

The days between those two dates are the total shelf life. The days between today and the expiration date are the remaining shelf life.

How Remaining Shelf Life Is Found

The math is simple subtraction:

Total shelf life = expiration date − manufacture date
Days elapsed = today − manufacture date
Days remaining = expiration date − today
Percent remaining = (days remaining ÷ total shelf life) × 100

The percent is often more useful than the day count. A product with 30 days left out of 60 is in much better shape than one with 30 days left out of 730.

Why the Percent Matters

Stores, hospitals, and factories buy and sell by percent of shelf life left. Many buyers will not accept a shipment unless it has at least 75% of its shelf life remaining. Stores often pull items from the shelf near the 10% mark. Warehouses use these numbers to decide what to ship first, what to mark down, and what to send back.

Common Shelf Life Rules

FEFO (First Expired, First Out): use or ship the item with the closest expiration date first.
The 75% rule: many retailers reject stock with less than 75% shelf life left on arrival.
The 50% point: the halfway mark, a good time to check stock levels and plan reorders around your supplier's lead time.
The 10% point: near expiry. Time to discount, use up, or donate.

Things That Change Shelf Life

The printed date assumes normal storage. Heat, light, damp air, and open packages make products go bad faster. Once you open a bottle or bag, the clock often speeds up, and the label may give a shorter "use within X days after opening" time. Store items in a cool, dry, dark spot to get the full shelf life.

Expired Does Not Always Mean Unsafe

For many foods, "best by" is about taste and texture, not safety. For baby formula, medicine, and some dairy and meat products, the date is a real safety limit. Always follow the label, and throw out anything that smells, looks, or tastes wrong, even if the date has not passed.

Formulas used

Total shelf life (days)

T_{\text{total}} = \frac{t_{\text{exp}} - t_{\text{mfg}}}{86{,}400{,}000\ \text{ms/day}}

Days elapsed

D_{\text{elapsed}} = \frac{t_{\text{ref}} - t_{\text{mfg}}}{86{,}400{,}000\ \text{ms/day}}

Days remaining

D_{\text{remaining}} = T_{\text{total}} - D_{\text{elapsed}}

Percentage of shelf life used

\text{Used} = \frac{D_{\text{elapsed}}}{T_{\text{total}}} \times 100\%

Percentage of shelf life remaining

\text{Remaining} = \frac{D_{\text{remaining}}}{T_{\text{total}}} \times 100\%

Milestone date for a given percent remaining

D_{p\%} = t_{\text{mfg}} + \left(1 - \frac{p}{100}\right) \times T_{\text{total}}

Duration converted to a display unit

V_{\text{unit}} = \frac{D_{\text{days}}}{f},\quad f_{\text{days}}=1,\ f_{\text{weeks}}=7,\ f_{\text{months}}=30.4375,\ f_{\text{years}}=365.25

Progress bar fill (clamped remaining percent)

\text{Bar} = \min\left(100,\ \max\left(0,\ \text{Remaining}\right)\right)

Frequently asked questions

What is the difference between best by, use by, and sell by dates?

They mean different things:

Best by (or best before): a quality date. After it, food may lose flavor or texture but is often still safe.
Use by: the last day the maker says the product is at its best. On baby formula and some fresh foods, treat it as a safety limit.
Sell by: a message to the store, not to you. It tells staff when to pull the item. Food usually stays good for a while after it.

Only infant formula is federally date-regulated in the United States. Most other dates are set by the maker.

What does the open jar symbol with 12M mean on cosmetics?

That is the Period After Opening (PAO) symbol. It shows how many months a product stays good after you first open it. A jar marked 12M lasts 12 months from the day you open it. Common values are 6M, 12M, 18M, and 24M.

The PAO clock is separate from the printed expiration date. Whichever date comes first is the one to follow.

How do I find the manufacture date if it is not printed on the label?

Many products use a batch or lot code instead of a plain date. Common patterns:

Julian code: a 4 or 5 digit number like 25183, meaning day 183 of 2025 (July 2, 2025).
Coded letters: a letter for the month (A = January) plus a number for the year.
Plant + date: a plant code followed by MMDDYY.

If you cannot crack the code, call or email the maker with the full lot number. They can tell you the exact production date.

How do companies decide what expiration date to print?

They run shelf life tests. Samples are stored under normal conditions and checked over time for taste, smell, texture, color, nutrients, and germ growth. The date is set a bit earlier than the point where the product actually fails, to leave a safety cushion.

Many makers also use accelerated testing: they store samples in heat and humidity to make them age faster, then use that data to estimate real shelf life in weeks instead of years.

Does the shelf life clock restart after I open a package?

No, it usually gets shorter. Air, moisture, and germs get in once the seal breaks. That is why many labels add a line like "use within 7 days of opening" or "refrigerate after opening."

After opening, follow two rules at once: the printed expiration date and the after-opening window. Use whichever ends sooner.

What is the difference between FIFO and FEFO?

FIFO (First In, First Out) moves the oldest delivery first. FEFO (First Expired, First Out) moves the item with the closest expiration date first.

They are not always the same. A box that arrived last month may expire after a box that arrived today, if the newer box was made earlier. For dated goods like food and medicine, FEFO is safer because it follows the real expiration date, not the delivery date.

Does freezing food extend its shelf life?

Yes. Freezing at 0°F (-18°C) stops germ growth, so frozen food stays safe almost forever. Quality still drops over time from freezer burn and dryness.

Rough quality windows in the freezer: ground meat 3-4 months, whole chicken up to 1 year, steaks 4-12 months, bread 3 months. Freezing does not undo spoilage, so freeze food while it is still fresh.

How long does canned food really last?

Most canned food stays safe for years past the printed date if the can stays cool, dry, and undamaged. High-acid foods like tomatoes, fruit, and pickles keep good quality for about 12-18 months. Low-acid foods like beans, corn, and meat can hold quality for 2-5 years.

Throw out any can that is bulging, leaking, badly rusted, or that spurts when opened. Those are real danger signs no matter what the date says.

Is it safe to take medicine after the expiration date?

Expired medicine often loses strength, so it may not work as well. That is a real risk for drugs you depend on, like antibiotics, insulin, nitroglycerin, EpiPens, and inhalers. Do not use those past the date.

Liquid medicines, eye drops, and anything refrigerated break down faster than pills. When in doubt, ask a pharmacist and replace it.

Which foods never really expire?

A few pantry items last almost forever when sealed and kept dry:

Honey
White sugar and salt
White rice
Dried beans (they just take longer to cook)
Pure vanilla extract and hard liquor
White vinegar

They still carry a date, but it is a quality date, not a safety one. Moisture or bugs are the real threats, not time.

Why is percent of shelf life remaining better than days remaining?

Because it shows the product's true condition. Thirty days left on a 60 day yogurt means half the life is gone. Thirty days left on a 730 day canned soup means it is almost done.

Percent lets you compare very different products side by side and set one rule for all of them, like "mark down anything under 25% remaining."

What shelf life percentage do buyers usually require on delivery?

Most retailers and distributors want stock to arrive with a set minimum of shelf life left. Common thresholds:

75% for packaged food and grocery goods
50-67% for pharmacy and medical supplies
85-90% for short-life fresh items

Shipments below the agreed level can be refused or charged back to the supplier, so check the remaining percent before you ship.

Does storage temperature really change shelf life that much?

Yes. A common rule of thumb is that every 18°F (10°C) rise in storage temperature roughly doubles the speed of spoilage reactions. A product rated for one year in a cool room may last only a few months in a hot garage.

Light and humidity hurt too. Keep goods cool, dry, dark, and off the floor to get the full life the label promises.

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