Protek Multi-Purpose Wood Treatment, the brand we stock, recommends that their product covers about 10m² per litre. So for a 6 foot x 4 foot shed, you'd have a total wall surface area of around 120ft if your wall panels were all 6ft tall (we calculated it earlier). 120ft = 36m. At 10m² per litre, you'd need about 4 litres for a 6x4. Note that Protek advise a second coat (two coats are recommended for sheds) will use much less product than the first coat. We sell stain and protector in 1, 2.5 and 5 litre tubs. You can calculate how much you'd need depending on the size of your shed yourself.