Metal Costing Factsheet
Ring Sizes
Size
Diameter
in mm
A
12.00
B
12.40
C
12.80
D
13.20
E
13.60
F
14.00
G
14.40
H
14.80
I
15.20
J
15.60
K
16.00
L
16.40
M
16.80
N
17.20
O
17.60
P
18.00
Q
18.40
R
18.80
S
19.20
T
19.60
U
19.80
V
20.20
W
20.60
X
21.00
Y
21.40
Z
21.80
Z+1
22.20
Z+2
22.60
Z+3
23.00
Z+4
23.40
Z+5
23.80
Z+6
24.20
Calculating Blank Length
To work out the length that you need
for your ring or collet, take the diameter
of your stone, or finger and add the
thickness of the metal to it and then multiply
the total by Pi.
Example for a Ring
For a size L ring, to be made with 1.2mm
thick sheet: 16.4 + 1.2 = 17.6
Then multiply the result by Pi. A good approximate
figure for Pi is 3.142
So, the piece of metal that you need to make your
ring will be 17.6 x 3.142 which equals 55.29mm
You can cut your metal to 55.5
Example for a Collet
For a collet to fit a 4.5mm round stone,
to be made with .5mm thick sheet:
Add the stone's diameter to the sheet thickness
4.5 + 0.5 = 5
Then multiply the result by Pi
5 x 3.142 = 15.71mm
You can cut your metal to 15.7mm.
Metal Costing
This seems hard at first, but when you can do it, it is very useful!
First of all, you need to know the density of the piece of metal
that you are trying to cost. The density is directly related to
the weight, but you already know this. A block of cheese the
same size as a brick is much lighter than the brick is. This is
because the brick has a higher density or Specific Gravity than
the cheese has. It is exactly the same with different metals.
To say how dense a particular metal is we give it a Specific
Gravity. Some useful specific gravities are listed here:
Specifc Gravities
Silver, Sterling
Silver, Fine
9ct Yellow
9ct White
9ct Red
14ct Yellow
14ct White
10.30
10.50
11.20
12.60
11.10
14.10
12.90
18ct Yellow
18ct White
22ct DS
Fine Gold
Gw Platinum
Brass
Copper
Please continue to the next page to learn how to use this new
and exciting information.
15.55
16.10
17.80
19.30
20.00
7.80
7.00
Metal costing - Sheet or Square Wire
To find the weight of sheet or square wire:
First you multiply its thickness by its width by its length,
then you multiply the result by that metal's specific gravity,
then you divide the whole result by one thousand.
You now have the weight in grams.
9ct yellow gold Flat band ring, size L, 1.2mm thick, 4mm wide
You already have worked out how long your ring blank needs
to be; we worked it out on the ring size exercise on the previous page.
We'll take the width of the ring as 4mm for this exercise.
So, Multiply it's thickness by it's width by it's length,
1.3mm x 4mm x 55.29mm, which equals 287.50
Then, because you want the weight in 9ct yellow gold, you multiply
the result by the specific gravity of 9ct yellow,
which is, as you now know, 11.20
287.50 x 11.20 = 3220 which you then divide by one thousand:
3220 which equals 3.220 grams which you can take as 3.2 grams
1000
This means that you can now telephone a gold supplier to ask the current
price per gram for 9ct yellow sheet and figure out how much the metal to make
this ring would cost. Remember that prices from suppliers may have to have
VAT (tax) added on top. For this ring, based on the price for 9ct yellow sheet today,
the 9th of May 2000, which is £3.18 per gram before VAT, the cost would be:
3.2 x 3.18, which equals £10.18, to which you need to add VAT. You can add
VAT at 17.5% with the percent button on your calculator, or times it by 1.175,
which will give the result including VAT. 10.18 x 1.175 = £11.96
Metal costing - Round Wire
To find the weight of round wire:
First you find the area of it's end, then you multiply that by it's length,
Then you multiply the result by that metal's specific gravity, then you
divide the whole result by one thousand. This gives you it's weight in grams
For example: If you want to make a bangle from round wire, you can
wrap a bit of soft wire around your hand to see what length it would need
to be to fit you. For this example, we'll say it's 200mm long.
Then you have to chose what diameter (thickness) of wire that you'd like.
Let us say for this example that it is 3mm in diameter.
Silver bangle 3mm in diameter, to the notional wrist size of 200mm
First find the area of it's end. For a circle, you use the formula
Pi times the radius squared. The radius is half the diameter,
Take Pi as 3.142 or use the Pi button on your calculator.
Radius is half the diameter: 0.5 x 3mm = 1.5mm
Area of the end of the wire =
3.142 x (1.5 x 1.5)
Area of the end of the wire =
3.142 x 2.25
Area of the end of the wire =
7.0695 mm squared
Then you multiply the area of the end by it's length:
7.096 x 200 = 1419.2
Multiply that by the specific gravity of the metal that you want;
In this case silver:
1419.2 x 10.3 = 14617.76
Divide by one thousand to find the weight in grams:
14617.8 14.61776 grams.
1000
In Sterling Silver at 18.5pence per gram + VAT this works out to be £3.18