The form says 2 MB, your computer says 1.9 MB — the megabyte mismatch

2026-10-08

The upload form says maximum 2 MB. Your computer says the photo is 1.9 MB. You press upload, and the form tells you the file is too large. Nobody is lying — the two numbers were measured with different rulers, and right at the limit that difference decides whether the file goes through.

Two megabytes that aren't the same size

A byte is a byte. A megabyte, in everyday software, has two definitions:

• Decimal: 1 MB = 1,000,000 bytes, and 1 KB = 1,000 bytes.
• Binary: 1 MB = 1,048,576 bytes (1,024 × 1,024), and 1 KB = 1,024 bytes. Strictly speaking these are mebibytes and kibibytes (MiB, KiB), but a lot of software prints the binary value under the plain MB and KB labels.

The binary megabyte is about 4.9% bigger than the decimal one; at the kilobyte level the gap is 2.4%. Far from a limit nobody notices. A few percent under a limit, it is the whole story.

How a 1.9 MB file breaks a 2 MB limit

Take a form whose limit is decimal — 2,000,000 bytes — and a file of 2,040,000 bytes. In binary megabytes that file is 1.945 MB, and a display that rounds to one decimal shows it as 1.9 MB. It is 40,000 bytes over the form's limit while looking comfortably under it.

Two things stacked up there: the unit difference, and the rounding. A one-decimal readout hides up to 0.05 MB in either direction, so the closer you sit to a limit, the less a rounded number tells you.

It can also go the other way. A site that counts in binary is slightly more generous than a decimal readout suggests. The trouble is that you rarely know which convention a form uses — few of them say. The safe assumption is the stricter one: decimal.

Which ruler our size readout uses

PixelSquish shows sizes in binary units: a KB on our screens is 1,024 bytes and an MB is 1,048,576 bytes. Below 1 MB we show whole kilobytes; from 1 MB up, one decimal place. The same rule applies everywhere a size appears — each file's before-and-after line, the total line above the list, and the preview.

Because the binary unit is larger, our figures read slightly lower than a decimal count of the same file. Exactly 2,000,000 bytes shows here as 1.9MB. And a file we show as 1.9MB can be anywhere from about 1.94 to about 2.04 million bytes — on either side of a decimal 2 MB limit.

How much margin to leave

Read our numbers against these targets and the convention question stops mattering:

• 2 MB limit: aim for 1.8MB or less on our readout. Anything we show as 1.8MB is at most about 1.94 million bytes.
• 1 MB limit: 1,000,000 bytes is about 977 KB in our units, so aim for 950KB or less.
• 500 KB limit: 500,000 bytes is about 488 KB in our units — aim for 480KB or less.

If you want certainty rather than margin, look at the exact byte count: the file's Properties window on Windows and Get Info on a Mac both list the size in plain bytes next to the rounded figure. Compare that number with the limit written out in full — 2 MB as 2,000,000 — and there is nothing left to interpret.

Getting under the line

Our compressor has three presets — Smaller (62), Recommended (82) and High quality (94) — and a slider from 40 to 100 between them. For a photo a few percent over a limit, Recommended is usually enough on its own. It never changes the image's dimensions, so if you are far over the limit, quality alone may not get you there; shrinking the pixel dimensions first in the resize tool is the bigger lever.

One line worth knowing: if re-encoding cannot make the file smaller, you get your original back with "Already well optimised — kept the original". That means this particular file is not going to shrink by compression alone, and the dimensions are where the savings are.

The short version

• "MB" means 1,000,000 bytes to some software and 1,048,576 to others; rounding hides the rest.
• Our readout uses the binary unit, so it reads a little lower than a decimal count.
• Leave margin: 1.8MB for a 2 MB limit, 950KB for 1 MB, 480KB for 500 KB.
• For certainty, compare the exact byte count with the limit written in full.