# Calculating P values from statistical ratios

**Free web calculators **

The easiest way to compute a P value from a statistical ratio is to use this free GraphPad QuickCalc page. It can compute a P value from a *z*, *t*, *F*, chi-square or *r *value you enter.

**Calculations with older versions of Excel**

If you want to compute P values using older (pre 2010) Excel, use these functions:

P value from F |
=FDIST (F, DFn, DFd) |

P value from t (two tailed) |
=TDIST (t, df, 2) |

P value from Chi Square |
=CHIDIST (ChiSquare, DF) |

P value from z (two tailed) |
=2*(1.0-NORMSDIST(z)) |

**Calculations with newer versions of Excel**

If you want to compute P values using newer (2010 and later) Excel, use these functions:

P value from F |
=F.DIST.RT (F, DFn, DFd) |

P value from t (two tailed) |
=T.DIST.2T(t, df) |

P value from Chi Square |
=CHISQ.DIST.RT(ChiSquare, DF) |

P value from z (two tailed) |
=2*(1.0-NORM.S.DIST(z,TRUE)) |

**Algorithms used in Prism**

GraphPad Prism (and InStat) report exact P values with most statistical calculations. It computes a P value from an F ratio using these algorithms, adapted from section 6.14 of the third edition of Numerical Recipes, which is available on line.

PFromF(F_Value, DF_Numerator, DF_Denominator) =

BetaI(DF_Denominato /2, DF_Numerator/2, DF_Denominator / (DF_Denominator + DF_Numerator * F_Value))

PFromT(T_Value, DF) = BetaI(DF /2, 1/2, DF / (DF + T_Value^2))

PFromZ(Z_Value) = PFromT(Z_Value, Infinity)

PFromR(R_Value) = PFromT(|R_Value| / SQRT((1 - R_Value^2)/DF) , DF)

PFromChi2(Chi2_Value, DF) = GammaQ(DF / 2, Chi2Value /2)

Note that BetaI is the incomplete beta function, and GammaQ is the incomplete gamma function.