how much, and what type, of top soil/compost is needed for a raised bed?

Compost is sold in litres e.g 60 litre bags
Measure length, breadth, height of the bed in cms.
Now multiply these three figures together. That will be the volume in cms cubed.
Divide by 1000.
1 litre = (10 cm)3 = 1000 cubic centimetres

That should give you the number of litres of compost you need. It will settle a bit once you put it in.
I would use the kind that does not contain peat as peat makes me feel guilty about peat bogs being mined to meet the whims of gardeners.

I hope she has good drainage for the bed, as the patio beneath could cause waterlogging. Is she going for decorative plants or fruit and veg.? Either way it will be a fun project, hope she enjoys it.

Sorry, just re-read OP, herbs etc will be great, but drainage especially important.

Just to enrich the soil a bit more, I first add some well rotted manure, if available locally. I would spread a thin layer of well rotted horse manure across the earth first, lightly fork it in, then add your compost. It must be well rotted, not fresh, otherwise it will damage the plants, if you are going to be planting soon.

Just a thought though as you mentioned herbs, some herbs like a free draining quite poor soil - ie the Mediteranean herbs -lavender, thyme, oregano etc. In this case I'd leave out the manure and add some grit or gravel to your compost.

However, Basil likes it rich, so maybe plant it up the other end, or in a pot sunk in the earth. Also do this with mint or it will take over the whole bed!

Good luck.

Thanks you for your replies.

What we really need to know is the proportions of compost/topsoil,sharp sand etc. We know that he needs about 900 litres but should it be all compost or should we add something else? We've looked on the internet but everything we can find assumes that the raised bed is on soil and not on a patio as is this one. There are gaps between the slats and at the bottom for drainage onto the patio which can then drain away.

Matt's flat only has a patio so we don't have the option of building it on soil and the raised bed is quite tall so he can tend the plants on a chair or stool and he is aiming to put a dwarf ornamental cherry in one corner.

Jess we only use peat free compost.

Just dawned on me that, not only did I miss the capital letter but I phrased the question badly.

By 'how much' I really meant how much of each component. I did go to school honest!!

oh well the maths workout over my porridge did not do my brain any harm.
Seeing as he is going for a mixed planting, no particular soil type. Parsley grows well in average stuff. (i reckon you can never have too much) and thyme is pretty difficult to kill. chives too. pretty and they will grow in almost anything.
Make sure he goes for a really miniature "rootstock" on anything tree like.

Thanks Jess we did the maths twice. DH was worried about trying to get it all in the car and round to the flat. We've bought an new sturdy wheelbarrow just for the job.

That is a lot of bags of compost.

I have manger type raised beds on my patio, I put loads of broken up polystyrene in the bottom therefore needing less compost, I grow flowers in mine, herbs could grow in the same way, I have noticed even though my garden is very open that a lot of plants have survived there over winter unlike the same type in the garden, could the polystyrene be generating some warmth?

Crikey; there's a thought. I've just cleared out a load of polystyrene from some pots that I'm planning on replanting [I'd read that pots don't need drainage and it's better to give the plants more soil] but I've always kept polystyrene for that purpose [especially as some of the pots are quite large and take a lot of filling. The plants in them seem to do very well; plants like hebes suvive the winter and annuals seem to go on until winter time [I'd put that down to the changing weather]. Am now going to put the polystyrene back in just in case .

I am glad I make all my own compost - 4x3x1.50m beds filled after 3 years here (and lots of horse manure to help the process and quality)- pheeew.

Tegan The polystyrene is likely to be a good insulator but does not generate any heat. I would worry a bit though that polystyrene is just not biodegradable and could contain some unpleasant chemicals. It does not look very good in the soil either.

Kitty The sort of compost you need depends on what you want to grow. I would Google the planting needs of what you want to grow first.

The "Wrong sort of Compost" often only shows its unsuitabilty after a few years when the plants give up trying. Different plants can be particulary sensitive to an Acid or an Alkaline soil. Some are very fussy about the drainage of the soil. Some need quite soggy ground others plants will curl up and die if the soil is not really dry.

Can you come and do mine when you have finished please.

<a href="http://www.topsoilshop.co.uk/topsoil-calculator">topsoil calculator uk</a>

Measure up your bed and just use a topsoil calculator uk? Its important you use one that is a UK based site so the measurements are correct.