Network Speed Calculator
File size, speed and time: the sum three ways round, the eight that everyone forgets, and why the real download always takes longer than the answer you calculated.
Revise this, the fun way
Play it interactively, earn XP and build a streak, free.
Start revising freeWhat you'll cover
How long will it take? ⏱️
A download is a division sum: the size of the file, divided by the speed of the connection. Two minutes of work, and every exam board asks it. There are only two ways to get it wrong, and this module is both of them. The first is the **unit slip** that turns a right method into a wrong number. The second is subtler: the sum gives you the **best possible** time, and a real transfer is always slower. A grade-9 answer knows both.
Two different units ⚖️
Almost every lost mark on this topic comes from one fact: **file sizes and connection speeds are not measured in the same thing.**
Where does it go wrong? ❓
A student divides a file size in megabytes by a speed in megabits per second, and writes the answer down. What has gone wrong?
- The two quantities are in different units, so the file size must be converted to bits first
- Nothing; megabytes and megabits are the same size
- They should have multiplied the size by the speed instead of dividing
- The answer needs converting from seconds into minutes
One relationship, three questions 🔀
Everything on this topic comes from a single relationship between **file size**, **speed** and **time**: **time = size / speed** - how long will this take? **speed = size / time** - how fast was that connection actually running? **size = speed x time** - how much could I transfer in that long? **Convert before you divide.** Get the file size and the speed into the same unit first - almost always by turning megabytes into megabits, multiplying by 8 - and only then do the arithmetic. If the answer comes out in bits and you were asked for bytes, divide by 8 at the end.
How long? 🧮
An interactive activity.
How fast was it? 📶
An interactive activity.
How much fits? 💾
An interactive activity.
Order the method 🪜
An interactive activity.
Why the real download is slower 🐢
Your calculation gives the **best case**. Several things make the real transfer take longer, and naming them is what separates a full-mark answer from an arithmetic one:
Why did it take longer? ✅
A calculation says a download should take 30 seconds; it actually takes 50. Select the TWO statements that could explain the difference.
- Other devices in the house were using the same connection at the same time
- Some of the data transferred was addressing and error checking rather than the file itself
- The file must have been larger than stated
- The connection was measured in bits rather than bytes
Diagnose it 🧭
An interactive activity.
The summary 📝
File sizes are measured in bytes and connection speeds in _____ per second, and there are _____ bits in a byte. To find a time you divide the size by the _____, having converted both to the same unit first. To find how much could be transferred you _____ instead. And the real transfer is always slower than the calculation, because of latency, protocol overhead and _____, where the bandwidth is shared with other users.
Your turn ✍️
An interactive activity.