The linear static problem in Ω⊂R3 for a thin shell made from an elastic homogeneous and isotropic material is analyzed. Let the shell thickness 2ϵ be constant, the neutral shell surface S:=θ(ω) be bounded connected and elliptic and the shell edge Γ be clamped. The solution u(ϵ) of the three-dimensional problem, the solution ζ of the corresponding two-dimensional membrane problem and the solution ζ(ϵ) of Koiter's two-dimensional model are compared with each other. The following three estimates are proved for ϵ small enough in the case when the mapping θ(ω) and the boundary γ:=∂ω are of class C4:||ζ(ϵ)−ζ||H1(ω)×H1(ω)×Hs(ω)≤Cϵ1/5−s/2,s∈[0,2],||u(ϵ)−ζ||H1(Ω)×H1(Ω)×Hs(Ω)≤Cϵ1/6−s,s∈[0,1],||u(ϵ)−ζ(ϵ)||H1(Ω)×H1(Ω)×L2(Ω)≤Cϵ1/6−s,s∈[0,1],where in the last two inequalities ζ and ζ(ϵ) are extended to Ω by letting ζ(y,x3)=ζ(y) and ζ(ϵ)(y,x3)=ζ(ϵ)(y) for all (y,x3) ∈ Ω:=ωx(-1,1).