Disk, card and connection speed
Measures how fast your SD card or drive reads, your upload and download speed, then tells you how long the transfer will take before you start it.
Bits and bytes, once and for all
1 byte = 8 bits · so Mb/s ÷ 8 = MB/s
Networks are sold in bits per second because the number looks eight times better. Files are measured in bytes. Every transfer estimate that turns out catastrophically wrong is this factor of eight, and it is worth memorising the handful of cases you actually meet:
| Sold as | Really | 1 GB takes | 100 GB takes |
|---|---|---|---|
| 10 Mb/s | 1.25 MB/s | 13 min 20 s | 22 h 13 min |
| 50 Mb/s | 6.25 MB/s | 2 min 40 s | 4 h 27 min |
| 100 Mb/s | 12.5 MB/s | 1 min 20 s | 2 h 13 min |
| 500 Mb/s | 62.5 MB/s | 16 s | 26 min 40 s |
| 1 Gb/s | 125 MB/s | 8 s | 13 min 20 s |
Upload is usually far slower than download on a domestic line, and it is upload you need when you deliver. Check the figure your provider quotes for upstream, not the headline one.
What the markings on a card actually promise
The big number on the front is a peak read speed, held briefly, under ideal conditions. It is not what you get when you offload a day of rushes. The small logos are the honest ones, because they are guaranteed minimum sustained write speeds — the only figure a manufacturer commits to.
| Marking | Guaranteed minimum write | What it is enough for |
|---|---|---|
| Class 10 / U1 | 10 MB/s | HD, and photography |
| U3 / V30 | 30 MB/s | 4K on most mirrorless cameras |
| V60 | 60 MB/s | 4K in a light raw or 10-bit codec |
| V90 | 90 MB/s | Heavy codecs, high frame rates |
A camera that drops frames on a fast-looking card is almost always meeting a sustained-write floor, not a peak. Match the V rating to the codec's data rate and the problem disappears.
Why there is no upload test here
Measuring an upload means uploading. There is no way around it: the only honest test of a connection is to actually push megabytes through it, to somebody's server. This site has no server-side component and does not want one — so it will not pretend to measure something it would have to break its own promise to see.
What it can measure without sending anything is the read speed of the card or drive in front of you, by timing a file you already have. That is the number people usually want anyway: offloading rushes is a read problem far more often than a network one.
Frequently asked
Why does my upload take eight times longer than I calculated?
Because you mixed bits and bytes. A connection is sold in megabits per second (Mb/s) and a file is measured in megabytes (MB). There are eight bits in a byte, so a 100 Mb/s line moves 12.5 MB per second, not 100. This one confusion is behind most of the “it said twenty minutes” evenings in this industry.
How can a web page measure my card without uploading anything?
It reads a file you point it at and times the read, chunk by chunk. The data goes from the card into your browser's memory and is thrown away immediately — it never leaves the machine, and there is nowhere for it to go: this page has no server-side component at all.
Why was my second measurement three times faster?
Your operating system cached the file. The second read came from RAM at several gigabytes per second, which tells you about your memory and nothing about your card. Measure a file you have not opened since you plugged the card in, and measure it once.
Why will you not measure my upload speed?
Because measuring an upload means actually uploading megabytes to somebody's server, and the whole promise of this site is that nothing leaves your browser. We would rather tell you that than quietly break it for one number. Your provider's own speed test will give you the figure; bring it back here and this page will do the arithmetic.
Is 1 GB a thousand megabytes or 1024?
For a card, a drive or a connection: a thousand. Manufacturers have always counted in powers of ten, which is why a “128 GB” card holds 128,000,000,000 bytes. Your operating system may then divide by 1024 and display 119 GB — same card, different arithmetic. This tool uses the manufacturers' convention so the numbers match what is printed on the box.
Why is the real write speed slower than the number on my card?
The number on the card is a peak, usually a read speed, held for a short burst. What matters when you are offloading is sustained speed, and it is often half. The V30 / V60 / V90 marking is the useful one: it is a guaranteed minimum sustained write of 30, 60 or 90 MB/s — the only figure a card maker commits to.