When you copy a formula, its cell references shift. Usually that's what you want. Sometimes it isn't: if every row needs the same VAT rate from one cell, that reference must stay put.

Relative and absolute references

You write	Copied one row down	Copied one column right
`B4`	`B5`	`C4`
`$B$4`	`$B$4`	`$B$4`
`$B4`	`$B5`	`$B4`
`B$4`	`B$4`	`C$4`

A $ locks the part after it. $B$4 is absolute: it never moves. Most of the time you'll want either no $ or both.

Shortcut: click into a reference in the formula bar and press F4 (Windows) or ⌘T (Mac Excel) to cycle through the four versions. In Google Sheets it's also F4.

Why keep rates in their own cell

Putting the VAT rate in one cell — instead of typing 0.05 into every formula — means you change it once and every price updates. The same goes for exchange rates, commission percentages and hourly rates. A rule worth keeping: if a number might ever change, give it its own labelled cell.

Percentages

5% and 0.05 are the same number. Format the cell as a percentage (Format → Number → Percent) so it reads clearly, and multiply by it as normal: =B7*$B$4.

PRACTICE — TAB "2 REFERENCES"

In C7, calculate the VAT: =B7*$B$4. Copy it down to C10.
In D7, add VAT to the price: =B7+C7. Copy down.
Check: the website's price including VAT should be 4,095.00.
Change the VAT rate in B4 to 10%. Every row should update. Then try removing the $ signs from C7 and copying again — see what breaks.