This invited paper presents two tools developed by the authors. Their purpose is to help the user in writing proofs regarding computer arithmetic, e.g., certifying a bound on a round-off error, while aiming at a high level of guarantee. Flocq is a library of mathematical definitions and theorems for the Coq proof assistant; Gappa is meant to compute bounds of values and errors, while producing the corresponding formal proof. We describe here these tools, how they interact and how they fit in a larger verification process.