Data Transfer Calculator

What the transfer time is made of

A data transfer runs two clocks at once. The first is the payload divided by the speed of the route. The second is the cost of handling each file, which is paid once per file no matter how small that file is.

The payload clock is one division: seconds = gigabytes x 1,000 / megabytes per second. A gigabyte here is 1,000 megabytes, the decimal definition that drive makers and phone settings use. The NIST table of binary prefixes keeps that decimal gigabyte separate from the gibibyte, which is 1,024 mebibytes. A folder Windows reports as 119 GB is 128 GB in this arithmetic, and that gap alone has cost people an hour of confusion on a full phone.

The file clock is the part most estimates leave out. Every file needs a directory entry, a permission check, a thumbnail and a closing handshake. On a phone to phone cable that costs about 0.15 seconds per file. On a cloud service the per file cost is higher, near 0.6 seconds, because each file makes a round trip to a server. A folder of 24,000 photos therefore pays 3,600 seconds, or one hour, before the last photo moves.

The calculator on this page adds both clocks. That is why the number it returns for a phone migration is often far larger than the number you get from dividing gigabytes by megabytes per second.

Route speeds, from USB 2.0 to Thunderbolt

Every route has a label speed and a delivered speed, and the gap between them runs from 30 to 80 percent. The label is the signaling rate of the link. The delivered rate is what a file copy reports after protocol overhead, error correction and the slower of the two devices.

The USB rows follow the USB 3.2 specification, which sets the three SuperSpeed rates at 5, 10 and 20 Gbps. The 40 Gbps row is Thunderbolt 3 and USB4, the fastest connector most laptops carry. A 64 GB folder is 64,000 MB, so at 400 MB/s it needs 160 seconds and at 40 MB/s it needs 1,600 seconds.

Two rows surprise people. Bluetooth stays on the list because setup screens still offer it for contacts and settings: it is fine for 2 MB of contacts and hopeless for a photo library. The cloud row sits last because a cloud move is two transfers, not one. The data goes up to a server and then down to the new device.

Worked example: 128 GB from an old phone to a new one

Take a phone with 128 GB in use: 24,000 photos and videos at about 5 MB each, plus apps and messages. The new phone arrives, the cable is USB-C to USB-C with USB 3.2 Gen 1 wiring, and the copy runs at 400 MB/s.

• Payload: 128,000 MB / 400 MB/s = 320 seconds, or 5 min 20 s
• Files: 24,000 x 0.15 s = 3,600 seconds, or 60 minutes
• Total: 3,920 seconds, or 1 hour 5 minutes

The payload is 8 percent of the total. That is the most useful fact about phone migrations: cable speed stops mattering once a folder holds tens of thousands of small files. A USB 2.0 cable at 40 MB/s would push the payload to 3,200 seconds and the total to 1 hour 53 minutes, so a faster cable saves 48 minutes on a job that still runs close to two hours.

The file count is the lever that moves the number. Copy the same 128 GB as 400 video files instead of 24,000 photos and the per file cost drops from 3,600 seconds to 60 seconds. The total falls to 6 min 20 s, a 10 fold difference from the same bytes on the same cable.

Free space matters as much as speed. A 128 GB transfer needs 128 GB free on the target before it starts, because most migration tools copy first and verify second. A new phone with 128 GB total storage cannot hold a 128 GB library plus its own operating system, and the copy fails at the end after an hour of work.

Small files cost more than their bytes

A 4 MB photo and a 4 MB slice of a video file carry the same payload and different overhead. The photo is a file. The slice is part of one. That difference decides how long a folder takes.

Ten thousand files at 64 GB works out to 6.4 MB per file, which is a normal photo library. That copy takes 27 minutes 40 seconds, and 25 minutes of it is overhead. At 100,000 files the job runs past four hours for the same 64 GB.

Three things cause the overhead. The file system updates a directory entry for each file, the operating system checks permissions and timestamps, and the receiving app rebuilds a media index so the gallery can show the new photos. The last one often continues after the progress bar reaches 100 percent, which is why a phone can feel slow for an hour after a migration claims to be finished.

When you control both ends, send an archive instead of loose files. A zip of 24,000 photos moves as one file and unpacks on the other side at disk speed, which turns a 65 minute job into roughly 10 minutes.

The cloud route moves every byte twice

A cloud backup is not one transfer. The old device uploads to a server and the new device downloads from that server, so the payload crosses your internet connection twice and the time is the sum of two connections that rarely run at the same speed.

Home upload is the slow half. Cable and fiber plans sell 200 to 1,000 Mbps down and 10 to 35 Mbps up, and the upload number decides when the backup finishes. A 50 GB library at 20 Mbps up needs 5 hours 33 minutes to leave the house, then 22 minutes to arrive at the new phone on a 300 Mbps download.

The FCC broadband speed guide lists 10 Mbps as the minimum for file downloading, and that is a floor for one activity at a time, not a promise for a 50 GB backup running in the background.

Metered data is the second cost. A cloud move counts the upload on the old device and the download on the new one, so a 50 GB library uses 100 GB of plan data when both devices sit on cellular. On a 20 GB hotspot that is five months of allowance spent in one afternoon. A cable or a direct Wi-Fi transfer counts nothing against a plan, which is the main reason to prefer one for a large move.

Wi-Fi Direct, AirDrop and Quick Share

The wireless handoff tools on phones skip the router and open a direct link between the two devices. AirDrop on Apple hardware, Quick Share on Android and Wi-Fi Direct in the older Android stack all work this way. They are convenient, and they run four to eight times slower than a cable.

Expect 40 to 90 MB/s from a modern pair of phones a few feet apart. The link is a shared radio, so both devices transmit on the same channel and every other network in range competes for airtime. Distance and walls cut the rate quickly: at 10 feet through one interior wall, a Wi-Fi 6 pair often drops under 30 MB/s.

Battery is the other limit. A 64 GB wireless transfer keeps both radios busy for 12 to 20 minutes and can take 10 to 15 percent off a phone battery, which matters when the old phone is the one with the tired battery. Plug both devices in, keep both screens awake and stop either one from locking, because a locked phone can pause the transfer without telling you.

There is one case where wireless wins. A phone with a charge only USB-C port and a worn connector can fail halfway through a 100 GB cable copy, and a direct Wi-Fi transfer at 55 MB/s finishes in 30 minutes without touching the port. Slow and finished beats fast and broken.

Data caps, hotspots and metered connections

A local transfer over a cable or a direct Wi-Fi link uses no plan data at all. The two devices sit inches apart and the bytes never touch a carrier network, so a 200 GB phone migration costs nothing on your bill.

A cloud transfer does touch it, twice, and carriers count both directions. The decimal gigabyte matters here too: a 20 GB plan is 20,000 MB by the carrier's count, and a phone reporting 18.6 GiB of photos holds 20 GB by that same count.

Hotspot data is the trap. Many unlimited phone plans carry a separate hotspot allowance of 15 to 50 GB, and a laptop backup over that hotspot draws from the smaller number. Read the hotspot line, not the headline plan, before starting a 64 GB cloud backup from a tethered laptop.

One more number belongs here. Throttling after a cap is usually 128 to 600 kbps, which is 0.016 to 0.075 MB/s. At 0.05 MB/s a 5 GB file needs 27 hours 47 minutes, so a throttled connection turns a cloud migration into a multi day project.

How to get a long transfer to finish

Most failed transfers fail for boring reasons, and each one has a cheap fix.

• Check the cable. Many USB-C cables that ship with a charger carry USB 2.0 wiring only, so a 5 Gbps port runs at 40 MB/s. A cable rated for 5 or 10 Gbps is marked on the connector or the packaging.
• Free the space first. The target needs the full payload free, plus 5 to 10 percent for the media index and thumbnails.
• Keep both devices awake and plugged in. A locked phone can pause a copy, and a phone that sleeps mid file leaves a partial transfer that most tools will not resume.
• Copy the big folders first. Videos finish fast and prove the link works before you commit an hour to a photo library.
• Split anything over 100 GB into batches. A failed 200 GB job restarts from zero on most tools, while a failed 50 GB batch costs 20 minutes.

After the copy, check the file count, not the size. A folder with 24,000 files on the old phone should show 24,000 on the new one. Total gigabytes can match while a few hundred files are missing, because a handful of large videos can hide a gap in a photo library, and because both devices round the total.

Delete the old copy only after the new one opens. A photo that shows as a gray thumbnail on the new phone is a file that copied badly, and it is the only warning you get before the source is wiped.