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Ref. No.: lSO/R 370-1964 (E)
IS0
STA ND AR D I Z AT I O N
I NT ERN AT I O NA L O RG A N I Z AT I O N FO R
IS0 RECOMMENDATION
R 370
CONVERSION OF TOLERANCED DIMENSIONS
FROM INCHES INTO MILLIMETRES AND VICE VERSA
1st ED IT10 N
May 1964
COPYRIGHT RESERVED
The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized tberefore oniy
by the national standards organization of that country, being
a member of ISO.
For each individual country the oniy valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
The IS0 Recommendation R 370, Conversion of Toleranced Dimensions from Inches
into Millimetres and Vice versa, was drawn up by Technical Committee ISO/TC 3, Limits
and Fits, the Secretariat of which is held by the Association Française de Normalisation
(AFNOR).
Work on this question by the Technical Committee began in 1953, taking into account
the studies which had been made by the former International Federation of the National
Standardizing Associations (ISA), and led, in 1962, to the adoption of a Draft IS0
Recommendation.
In May 1962, this Draft IS0 Recommendation (No. 510) was circulated to all the
IS0 Member Bodies for enquiry. It was approved, subject to a few modifications of an
editorial nature, by the following Member Bodies:
Australia Germany Norway
Austria Greece Poland
Belgium Hungary Portugal
Burma India Rom ani a
Canada Iran Spain
Chile Ireland Sweden
Czechoslovakia Italy Switzerland
Japan Turkey
EDPt
Finland Netherlands United Kingdom
France New Zealand Yugoslavia
Two Member Bodies opposed the approval of the Draft:
U.S.A., U.S.S.R.
The Draft IS0 Recommendation was then submitted by correspondence to the
IS0 Council, which decided, in May 1964, to accept it as an IS0 RECOMMENDATION.
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I
IS0 / R 370 - 1964 (E)
CONTENTS
Page
1. Purpose . . 5
... 5
2. Generalities .
... 6
...............
3. Conversion of inches into millimetres
... 7
...............
4. Conversion of millimetres into inches
... 8
5. Special methods of application .
Appendix : Conversion tables
1. Tables for conversion from inches into millimetres
1.1 Inches in fractions . 9
1.2 Inches in decimals and inches . 10
2. Tables for conversion from millimetres into inches . 11
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IS0 / R 370 - 1961 (E)
IS0 RECOMMENDATION R 370 May 1964
CONVERSION OF TOLERANCED DIMENSIONS
FROM INCHES INTO MILLIMETRES AND VICE VERSA
1. PURPOSE.
The present'IS0 Recommendation concerns the conversion of toleranced dimensions from inches
into millimetres and vice versa.
The object of the IS0 Recommendation is to define, for agreements which refer to it, methods
of conversion which ensure, under the most suitable conditions, practical regard for interchange-
ability corresponding to the specified tolerances.
2. GENERALITIES
2.1 The use of the conversion factor 1 in = 25.4 mm exactly * generally produces converted
.
values containing more decimal places than are required for the desired accuracy.
It is therefore necessary to round these values suitably, in accordance with the degree of
accuracy of the toleranced dimensions, which depends on the magnitude of the tolerance
specified.
2.2 Application of the rules for rounding, as given in Tables 1 and 2 (clauses 3 and 4 of this
IS0 Recommendation), guarantees that even in the most unfavourable extreme cases neither
of the two specified limits will be exceeded by more than 2 to 2.5 per cent of the value of
the tolerance.
2.2.1 In Method A, which is generally applicable, unless expressly indicated to the con-
trary, the rounding is effected to the nearest rounded value so that, on the average,
the converted tolerances remain statistically identical with the original tolerances.
The limits converted by this method are considered acceptable for interchange-
ability and serve as a basis for inspection.
2.2.2 In Method B, rounding is effected systematically towards the interior of the tolerance
zone so that, on the average, the converted tolerances are smaller than the original
tolerances.
L
Consequently this method should be employed only when, by special agreement, the
original limits have to be respected absolutely (in particular, when components are to
be inspected by means of original ** gauges).
These two methods form the subject of clauses 3 and 4 of this IS0 Recommendation which
relate respectively :
clause 3, to the conversion of inches into millimetres
clause 4, to the conversion of millimetres into inches
and are supplemented :
clause 5, by various special methods of application.
Finally, there is an appendix containing conversion tables from inches into millimetres and
vice versa, on the basis of the conversion factor 1 in = 25.4 mm.
* See IS0 Recommendation R 31, Part I, Fundamental Quantities and Units of the MKSA System and Qirantities and
Units of Space and Time, No. 1-3.h.
*+ Gauges in the dimension to be converted.
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iSO/ R 370-1964 (E)
3. CONVERSION OF INCHES INTO MILLIMETRES
3.1 Method A (General Rule)
(a) For each dimension in inches, consider only its two limits, maximum and minimum;
(b) Convert the corresponding two values exactly into millimetres by means of the con-
version factor: 1 in = 25.4 mm (see the conversion tables in the appendix, pages 9 and 10);
(c) Round the results obtained in this way to the nearest rounded value as indicated in
Table 1 below, depending on the original tolerance in inches, i.e. on the difference
between the two limits in inches.*
The use of this method guarantees that even in the most unfavourable extreme cases neither
of the two original limits will be exceeded by more than 2 per cent of the value of the
tolerance.
3.2 Method B (by special agreement)
As Method A, except that the rounding is not effected to the nearest rounded value, but
towards the interior of the tolerance (i.e. to the next lower value for the upper limit and to
the next higher value for the lower limit).
This method should be employed only when the original limits have to be respected ab-
solutely (in particular, when components are to be inspected by means of original gauges).
TABLE 1
Original tolerance in inches
Round off to a
whole number of
not less than 1 and below I
in 1 in
I mm
I I
o.oO0 O1
o.oO001 I o.oO01
0.0001 0.001 0.0001
0.001 0.01 0.001
0.01
0.01 o. 1
Example
Suppose that a dimension is expressed in inches as follows:
1.950 & 0.016 (=1.966, 1.934)
Conversion of the two limits into millimetres gives
49.1236 and 49.9364
As the tolerance equals 0.032 in and thus lies between 0.01 and 0.1 in, it is necessary, em-
ploying Method A, to round these values to the nearest 0.01 mm. The values in millimetres
to be employed for these two limits are thus
49.12 and 49.94
(Rounding towards the interior of the tolerance, according to Method B, would give
limits of 49.13 mm and 49.93 mm, i.e. a tolerance reduced to 0.80 mm instead of 0.82 mm,
as given by Method A.)
* This amounts to rounding each of the two values converted into millimetres to a whole number of 1 x IO-'' mm,
1 x IO-'' and less than 10 x IO-" in.
when the original tolerance in inches lies between
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I
lSO/ R 370-1964 (E)
4. CONVERSION OF MKLIMETRES INTO INCHES
4.1 Method A (General Rule)
(a) For each dimension in millimetres consider only its two limits, maximum and minimum;
(b) Convert the corresponding two values into inches by means of the table on page 11
(based on the conversion factor: 1 mm = 1/25.4 in)
(c) Round the results obtained in this way to the nearest rounded value as indicated
in Table 2 below, depending on the original tolerance in millimetres, i.e. on the diffe-
rence between the two limits in millimetres.*
The use of this method guarantees that even in the most unfavourable extreme cases neither
of the two original limits will be exceeded by more than 2.5 per cent of the value of the
tolerance.
4.2 Method B (by special agreement)
As Method A, except that the rounding is not effected to the nearest rounded value, but
towards the interior of the tolerance (i.e. to the next lower value for the upper limit and to
the next higher value for the lower limit).
This method should be employed only when the original limits have to be respected ab-
solutely (in particular, when components are to be inspected by means of original gauges).
TABLE 2
Original tolerance in millimetres
Round off to a
whole number of
not less than I and below
mm 1 mm in
0.0003 0.005 0.000 001
0.000 O1
0.005 0.05
0.05 0.5 0.0001
0.5 5 0.001
5 50 0.01
Example
Suppose i.at a I imension is expressed in millimetres as follows:
49.5 0.4 (= 49.1, 49.9)
Conversion of the two limits into inches gives
1.933 070 9 and 1.964 566 9
As the tolerance equals 0.8 mm, and thus lies between 0.5 and 5 mm, it is necessary, employ-
ing Method A, to round these values to the nearest 0.001 in. The values in inches to be
employed for these two limits are thus
1.933 and 1.965
(Rounding towards the interior of the tolerance, according to Method B, would give
limits of 1.934 in and 1.964 in, i.e. a tolerance, reduced to 0.030 in instead of 0.032 in as
given by Meth
...