joeleicester wrote: I`m not bothered about such a low volume I end up with for a first go in case its ends up tasting a bit crap

LOL Joe!

Don't get too hung up on your gravity figures for your first few brews but you do need a place to start. Mini-BIABs (those in a small pot) are a little harder to work out.

I'm not an expert in mini-BIAB's so I would firstly read Ralph's guide here.

Here's the way I would play it assuming you want a beer of about 5% ABV. This will save you doing anything like sparging etc.

1. Use about 3.3kgs of grain. (This is based on 60% efficiency as you are sort of (no-sparging.")
2. Half-fill your pot with water like Ralph says and then rain in your grain. Then add some more water at mash temp if you can.
3. Mash it for 90 mins and then pull your bag and give it a good squeeze.
4. Get your kitchen kettle going so you can keep topping up your kettle anytime there is space. Aim to use a total of 18 L on your brew day. This will include what you add to the mash of course. Anything you have not been able to add to the kettle during the boil, add to your fermenter.

If you do the above, I reckon you will end up with around 10 L of 5% beer into your fermenter. I have had to guess a bit but it is a bit of a guessing game with any brewing method (more so with mini-BIAB's)until you get a few brews under your belt so try and keep some good notes.

Working out what pre-boil gravity you need will not be helpful in your situation as you won't have added all the water you can. (With a bigger pot, this figure can be more useful and predictive as you are mashing etc with all your water from the start.)

Ralph might have some excellent corrections to the above but I think if you are brewing a robust recipe like an APA or a Schwartzbier then you won't go far wrong with the above. You'll still get a great beer. (If the OG is too high, dilute it with some more water in the fermenter. If it is too low, you will still have a very tasty, slightly less alcoholic brew.)

Let us know how you go Joe ,
PP

Quite right Pat, there's just a couple of things I can add- great tips though!

Firstly, good on you Joe for giving BIAB a whirl, its as easy as falling off a log! I agree with Pat, not getting hung up on the numbers is important, also that this BIAB AG method is quite forgiving, you have to try really hard to stuff things up. So relax, bung that 3-odd kilos of grain in and just mash it, see how it goes, chances are at the end of the day you'll have some excellent beer!

Now, specific gravity scales linearly, its easier if you can imagine that there's a fixed amount of solids and a variable amount of liquid, the only thing changing being the liquid (i.e. the volume), so it is just straight dilution. With known volumes it is like this:
Pre- boil Volume / Post- boil Volume * Pre- boil Concentration = Post- boil Concentration
In this (admittedly rough) formula, Specific Gravity is expressed as whole numbers (like degrees Plato), it is reasonably valid to just say 1.035 is equivalent to 35. So, for your situation:
12 / 9.5 * 35 = 44
I've found this works quite reliably at home brewing concentrations, however you'll have to know your evaporation rate inside out to predict these things with much confidence. I'd say that's in the ballpark though.

Now, I'm sorry if this is a bit of a digression, but it is related and will become clearer later on, (when my MaxiBIAB(TM) guide is finished!). One other really important thing is that it is much easier to dilute a slightly concentrated wort afterwards than to boil it for longer to get any excess water evaporated down to a particular specific gravity- and doing that will just increase the bitterness and wreck flavour and aroma additions. By adding more malt rather than less, you can ensure that if in the case of poor efficiency you get what you wanted, or alternately if things go well, just more beer. So, my advice it to be generous with your grain bill rather than frugal.
I know BIAB can be extremely efficient, but for the wont of a few pennies- worth more of malt, you can either be 100% sure the post- boil gravity is at or above your target at worst, or at best be able to dilute your concentrated wort to yield more beer than you'd estimated based on a lower efficiency. Either way, it is win- win.
Particularly with these smaller kettles, I favour post- boil dilution, it takes out a lot of the fuss and bother of hitting target specific gravities and volumes, plus if the planets line up for you, there's a good chance of getting more beer than you'd budgeted for, which is just fab in anyone's language.
Folks, I'm really sorry that my MaxiBIAB guide isn't quite ready yet, but I've actually covered much of it here. It will enable you to scrape big batches out of small kettles, but also to make brewday a breeze with no worrying about the numbers. PM me Joe if you want it pre- release, otherwise, scale the MiniBIAB to suit your 15L kettle.

Ps. Another essay, humblest apologies...

Sounds like a positive outcome Joe.

Your beer will clear, just give the yeast some time to do it's job.

"Patience Grasshopper"