What size back pack 20L or 30l?

Tiberius007

Any advice on what size backpack for a daily commute - I carry lunchbox, usual clothes for the day (shirt, sweater, trousers etc), towel, the usual essentials, tube, pump etc, and waterproof for our lovely unpredictable weather.
I am looking at fully waterproof/dry bags such as those by Over Board which seem good and get good reviews and will keep everything dry etc, just cannot gauge the size. 20L or 30L......?
Anyone else had the same problem? Any advice greatly appreciated.
Cheers

snig

30l defo.

but rather you than me, I use a topeak beam rack, carries 20llbs on the seatpost,much better than a back pack and easily removed when not needed.

I didn't like the sweaty back a backpack means.

CocoRide

Personally I think you'd be fine with a 20l but either will be adequate

Tiberius007

Thanks guys.
Agree re the sweaty back, and will look at the beam rack you mention for future ref.
That said I have gone for a 30L. It was an ex demo, so good price - which works out quite well as I can always buy the 20L also, if need be for shorter journeys with less to carry, as it works out the cost of the full price 30L that I may have bought.
Happy days.

Tiberius007

Thanks guys, as I said in the previous posting, I went for the 30L. Picked the bag up today and it is great. Polaris Aquanought (black or very dark grey).
Will fit everything in I want - I think the 20L would have been a bit of a squeeze.