The Speech Intelligibility Index (SII) is an often used calculation method for estimating the proportion of speech information available in noise. For speech reception thresholds (SRTs), measured in normal-hearing listeners using various types of stationary noise, this model predicts a fairly constant speech proportion (about 0.30) necessary for sentence intelligibility. However, when the SII model is applied for SRTs in quiet, the estimated speech proportions are often lower, and show a larger inter-subject variability, than found for speech in noise near the normal speech level (65 dB SPL). The present model attempts to alleviate this problem by including cochlear compression. It is based on a loudness model for normal-hearing and hearing-impaired listeners of Moore and Glasberg [Hear. Res. 188, 70–88 (2004)]. It estimates internal excitation levels of the speech and then calculates the proportion of speech above threshold using similar spectral weighting as used in the standard SII. The present model and the standard SII were used to predict SRTs in quiet for both normal-hearing and hearing-impaired listeners. The present model predicted a set of data for three listener types (normal hearing, noise-induced hearing loss, and age-induced hearing loss) with less variability than the standard SII.