How do you calculate if a block of wood will float in water or not?

If the density of the block is less than the density of water, then the block will float. Density of water is close to 1 gram per cubic centimeter. So measure the block and calculate its volume (Length x Width x Height). Use a scale to find the block's mass. Then divide mass/volume to calculate density. If you've measured in grams and centimeters, then the units will be g/cm³, then compare this to 1 g/cm³.