BACKGROUND Mean pulmonary artery pressure (mPAP) may be estimated by using the classic rule of thumb, namely 2/3 x dPAP + 1/3 x sPAP, where dPAP = diastolic PAP and sPAP = systolic PAP. Studies have suggested that mPAP may be also estimated from sPAP alone. Pulmonary hypertension (PH) is usually defined by an invasive mPAP > 25 mm Hg, but the corresponding sPAP threshold remains to be established. Our study evaluated the accuracy and precision of various empirical formulas relating mPAP and sPAP in resting adults. METHODS Five previously published studies with individual high-fidelity PAPs were analyzed (n = 166 individuals, 57% of whom had PH). The time-averaged mPAP was compared with formula one (F1), the classic rule of thumb; formula two (F2) = dPAP + 0.41 x pulse PAP; formula three (F3) = square root of (sPAP x dPAP); formula four (F4) = 0.61 x sPAP + 2 mm Hg; and formula five (F5) = 2/3 x sPAP (parabolic shape). RESULTS The mPAP ranged from 9 to 82 mm Hg and was related to sPAP (r(2) = 0.98). The most accurate formula was F4 (mean bias, 0.0 mm Hg). The most precise formula was F1 (SD of the bias, 1.6 mm Hg). Other formulas gave estimates of essentially similar accuracy, while F2 and F3 were more precise than F4 and F5. sPAP > 36 mm Hg could be used to diagnose PH (mPAP > 25 mm Hg) with a 97.9% sensitivity and 98.6% specificity. CONCLUSIONS In resting adults, the most accurate estimate of mPAP was obtained by using sPAP only, while the combination of sPAP and dPAP gave the most precise mPAP estimate. The sPAP threshold of 36 mm Hg could be used to diagnose PH with high sensitivity and high specificity.