It's exactly #1# #"g"/"cm"^3#. Here is an equation to calculate it at certain temperatures:
#rho_(adj) = (rho_(ref))/(1+beta(T_(adj) - T_(ref)))#
where:
#rho# is the density in #"g"/"cm"^3# at either some reference temperature (#10^oC# in this case) or some adjusted temperature (if you want to calculate for #4^oC#, that is #T_(adj)#)
#T# is the temperature, either the reference or adjusted one
#beta# is the expansion coefficient (for water it is 0.000088 #"cm"^3/("cm"^3*^"o""C")# on average from #0^oC# to #20^oC#)
For example:
#rho_(4^oC) ~~ (rho_(10^oC))/(1+0.000088(4^oC - 10^oC))#
#~~ (0.9997026 "g"/"cm"^3)/(1+(0.000088"cm"^3/("cm"^3*^"o""C"))(4^oC - 10^oC))#
#~~ (0.9982071 "g"/"cm"^3)/(1+(0.000088 "cm"^3/("cm"^3*^"o""C"))(4^oC - 10^oC))#
#~~ 1.0002 => 1.0 "g"/"cm"^3#
If we were to have accounted for the change in the density with pressure as well, it would have been exactly #0.999975#. I often use #0.9977735# for #22^oC# and #0.9970749# for #25^oC#.
http://antoine.frostburg.edu/chem/senese/javascript/water-density.html
