In space radiation calculations it is often useful to calculate the dose or dose equivalent in blood-forming organs (BFOs), the eye, or the skin. It has been customary to use a 5-cm equivalent sphere to approximate the BFO dose. However, previous studies have shown that a 5-cm sphere gives conservative dose values for BFOs. In this study we use a deterministic radiation transport with the Computerized Anatomical Man model to investigate whether the equivalent-sphere model (ESM) can approximate organ doses in space radiation environments. We have determined the organ-specific constant radius parameters and the corresponding average errors of using the ESM at those radius parameters. We find that for galactic cosmic ray (GCR) environments, the ESM with a constant radius parameter works well in estimating the dose and dose equivalent in BFOs, the eye, or the skin, and the average errors of using the ESM are all <2%. For solar particle event (SPE) environments, however, the radius parameters for organ dose or dose equivalent increase significantly with the shielding thickness, and the model works marginally for BFOs but is unacceptable for the eye or the skin. To estimate the dose equivalent in BFOs, for example, the constant radius parameter is determined to be ~10.5 cm for GCR environments and ~7.8 cm for SPE environments, and the corresponding average error of using these radius parameters in the ESM is 0.7% and 17%, respectively.