Using Bellman function techniques, we obtain the optimal dependence of the operator norms in L 2 (R) of the Haar multipliers T t w on the correspondingcharacteristic of the weight w, for t = 1, ±1/2. These results can be viewed as particular cases of estimates on homogeneous spaces L 2 (vdσ), for σ a doubling positive measure and v ∈ A d 2 (dσ), of the weighted dyadic square function S d σ . We show that the operator norms of such square functions in L 2 (vdσ) are bounded by a linear function of the A d 2 (dσ) characteristic of the weight v, where the constant depends only on the doubling constant of the measure σ. We also show an inverse estimate for S d σ . Both results are known when dσ = dx. We deduce both estimates from an estimate for the Haar multiplier (2 . The estimate for the Haar multiplier adapted to the σ measure, (T σ v ) 1/2 , is proved using Bellman functions. These estimates are sharp in the sense that the rates cannot be improved and be expected to hold for all σ, since the particular case dσ = dx, v = w, correspond to the estimates for the Haar multipliers T 1/2 w proven to be sharp.