所謂基準距離，就是螺紋用手旋合到的位置（保證密封性能），定為基準面，旋到基準面的距離就是基準距離。有了基準面，才有基準距離。
英制美制管螺紋標準及新舊對照
用螺紋密封的管螺紋基本尺寸（R）
尺寸代號 每英寸牙數   螺距 基準面上的基準尺寸   基準距離    有效螺紋長度  大徑   小徑
1/16          28             0.907    7.723    6.561   4.0                6.5
1/8           28             0.907    9.728                      8.566          4.0                6.5
1/4           19             1.337    13.157                   11.445           6.0                9.7
3/8           19             1.337    16.662                   14.950           6.4                10.1
1/2           14             1.814    20.955                   18.631           8.2                13.2
3/4           14             1.814    26.441                   24.117           9.5                14.5
1              11             2.309    33.249                   30.291           10.4             16.8
1¼           11             2.309    41.910                   38.952           12.7             19.1
1½           11             2.309    47.803                   44.845           12.7             19.1
2              11             2.309    59.614                   56.656           15.9             23.4
2½           11             2.309    75.184                   72.226           17.5             26.7
3              11             2.309    87.884                   84.926           20.6             29.8
3½           11             2.309    100.330                    97.372           22.2             31.4
4              11             2.309    113.030                    110.072       25.4             35.8
5              11             2.309    138.430                    135.472       28.6             40.1
6              11             2.309    163.830                    160.872       28.6             40.1

R—密封用管螺紋：Rc-圓錐內螺紋   Rp-圓柱內螺紋   R-圓錐外螺紋
G—非密封用管螺紋