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An $f(R)$ gravity model is proposed to realize a late time acceleratedexpansion of our Universe. To test the viability of an $f(R)$ gravity modelthrough cosmic observations, the background evolution and theEinstein-Boltzmann equation should be solved for studying the effects on thecosmic microwave background power spectrum and on the matter power spectrum. Inthe market, we already have the modified versions of {\bf CAMB} code, forinstance {\bf EFTCAMB} and {\bf MGCAMB}. However, in these publicly availableEinstein-Boltzmann codes, a specific background cosmology, for example the$\Lambda$CDM or $w$CDM, is assumed. This assumption would be non-proper for aspecific $f(R)$ model where the background evolution may be different from a$\Lambda$CDM cosmology. Therefore the main task for this paper is to present acode to calculate the anisotropies in the microwave background for any $f(R)$gravity model based on {\bf CAMB} code, i.e. {\bf FRCAMB}, where the backgroundand perturbation evolutions are included consistently. As results, one cantreat {\bf FRCAMB} as a blackbox to output the CMB power spectrum and matterpower spectrum, once an $f(R)$ function, its first two derivative with respectto $R$, i.e. $f_R\equiv df/dR$, $f_{RR}\equiv d^2f/dR^2$ and the reasonablevalues of the model parameters are inputted properly. As by-products, one canalso output the effective equation of state of $f(R)$ model, the evolution ofthe dimensionless energy densities and other interesting cosmologicalquantities.

FRCAMB: An $f(R)$ Code for Anisotropies in the Microwave Background