Thickness of plywood over floorboards

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Hi everyone. Hope you are well.
I am thinking of having vinyl flooring installed in an old terrace house bathroom with bare floorboards. Before I can do that I need a subfloor of plywood fitted. My question is what thickness of plywood would be ideal? Many thanks for any advice.
 
My questions would be, "How flat are your current boards?" and "Are there major gaps in your boards?". If your floor is relatively flat (no high or low boards) and you don't have big gaps between the boards you can get away with 6mm plywood with a skim of levelling compound over the top - although 12mm is preferable for many floors. 18mm is only necessary if you have a truly horrendous original floor, or you need to build the floor up to match, say, an engineered floor outside the door

Just bear in mind that whatever you put down will need an awful lot of fixings - nails (ring annular clouts) are driven at 100mm centres (325 per 8 x 4ft sheet), whilst screws can be driven at 150mm centres except for the rows along the edges, which should be set out on 100mm centres (or about 165 screws per sheet). If you screw make sure you sink the screws flush or just under the surface. Don't skimp on fixings
 
Thank you for your replies.

The floorboards are in quite a good condition and level with no big gaps between. Also I don’t want the bathroom floor to get higher than the landing which is carpeted. I am thinking 6 mm plywood.
 
If you don't want to raise the floor height, why not rip the floorboards up and lay 18mm ply and then leveling compound.
 
Give up said:
The floorboards are in quite a good condition and level with no big gaps between. Also I don’t want the bathroom floor to get higher than the landing which is carpeted. I am thinking 6 mm plywood.
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Your flooring (vinyl) often works out at about 3 to 4mm thickness. If you have a fairly flat floor to start with your skim would be about 2mm, so the sub floor needs to finish 5 to 6mm below the level of the carpet. You can hide a few millimetres of difference in level using a threshold ovr transition strip (available in metal and timber)

Flooring plywood often comes in 5.5mm (6mm), 8mm (only sometimes), 9.5mm and 12mm. Basically, for a bathroom you should should an exterior grade hardwood plywood.

My take on overboarding is that it is considerably faster, considerably cheaper and considerably less mess and trouble than ripping a full floor up and replacing it - especially given that doing so can involve you having to put extra supports into carryvany stud walls previously carried by the old planked floor. For many people that makes it a preferable option
 
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