福禄数（Froude number,Fr）为流体力学中无量纲的纯量，为惯性力和重力效应之比，公式如下：

式中U为流体速度，L为物体特征长度，g为重力加速度。

明渠流和波浪力学中都常用到福禄数。在明渠流中，长度L为水深h。在波浪力学中，福禄数代表平均流速与重力波的波速之比。

In continuum mechanics, the Froude number (Fr) is a dimensionless number defined as the ratio of the flow inertia to the external field (the latter in many applications simply due to gravity). Named after William Froude (/ˈfruːd/;), the Froude number is based on the speed–length ratio as defined by him, and is defined as: