Working out how much grout you need for a mosaic project isn't a straightforward question, it is dependent upon the following factors:
The surface area of your mosaic
The size of the gaps between your tiles
The thickness of the tiles
The amount of grout that is wasted.
I have a tendency to work based on touch and feel, so I have lots of grout available and never work out how much I need. I tend to waste grout because of this, but it is what works for me. If you are working on a large project or would like to get an accurate estimation on how much grout you need, Bostik have a grout calculator on their website: http://www.bostik.com.au/construction-trade-grout-calculator.html.
Wastage of Grout
When it comes to grouting there is potential to waste a lot of grout, think about the amount that is left of the sides of the mixing container, on the tools and gloves, or that ends up on the floor. Here are a few pointers to help you reduce the amount of wastage:
Lay a sheet of plastic over your work area before you begin to grout. This will not only protect your work surface but will allow you to scoop up any wet grout and apply it to the mosaic. Make sure it doesn't fall on the floor, as you will have to throw it out just in case it has become contaminated with dirt etc.
When mixing your grout, scrap down the sides of your container and keep the grout mixed together. This will help to stop the edges of the grout from drying out which them can't be used. A pallet knife is a great tool for this job.
A Few Grouting Pointers
When it comes to mosaicing, the cost of the grout is relatively inexpensive when compared to the cost of your materials and time. It is better to have too much grout than too little; it is really frustrating to get half way through grouting only to run out.
After you have mixed your grout, let it sit for 10 minutes or so before using it. This time is needed for the chemical reactions to take place. Once you've mixed your grout never add additional water to it.
It is important to keep your grout moist while it is curing. You can use a spray bottle to moisten it down while it cures.
Don't wipe your surface down with a really damp rag after grouting; it can remove some of the colour pigment from your grout. I only discovered this information recently and it makes so much sense. I use to clean off the excess grout with a really damp cloth, but couldn't work out why the grout would look 'speckled' afterwards. It's because the colour was being leeched out of the grout.
Always seal your mosaic when completed to help prevent against moisture.
When applying sealer, make sure you don't let it dry on your tiles. It will leave a residue that you can't remove.