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Summary: The different sizes of screws and the conversion from metric to imperial sizes.

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Screw sizes are a mystery to most people so we have tried to make life a little easier with the table below. A wood screw is sized by two different numbers. First is the diameter of the screw. The larger Phillips Head the number, the larger the diameter. A number 4 screw is much smaller than a number 12 screw. The length is given next and it should be remembered that the length given for a screw is the length that is buried in the wood or other material, it does not include the head of raised, or domed headed screws.

Slotted screws are fast becoming history and cross head screws are now the fashion but it must be remembered that a cross head screw can either be a Philips screw (Top image)or a Pozidrive Head

pozidrive/supadrive screw. They both require a different driver which can be found below.

See also our projects on different types of screw and Countersinking. Pilot holes should also be used when inserting screws.

Metric/Imperial Conversion Chart
Gauge mm	Length mm	Imperial Size	Gauge mm	Length mm	Imperial Size
3	12	4 x 1/2	3.5	12	6 x 1/2
	16	4 x 5/8		16	6 x 5/8
	20	4 x 3/4		20	6 x 3/4
	25	4 x 1		25	6 x 1
	30	4 x 1 1/4		30	6 x 1 1/4
	40	4 x 1 1/2		40	6 x 1 1/2
4	12	8 x 1/2	4.5	25	9 x 1
	16	8 x 5/8		30	9 x 1 1/4
	20	8 x 3/4		35	9 x 1 3/8
	25	8 x 1		40	9 x 1 1/2
	30	8 x 1 1/4		45	9 x 1 3/4
	35	8 x 1 3/8		50	9 x 2
	40	8 x 1 1/2		60	9 x 2 3/8
	45	8 x 1 3/4		70	9 x 2 3/4
	50	8 x 2		75	9 x 3
	60	8 x 2 3/8
	70	8 x 2 3/4
5	25	10 x 1	6	30	12 x 1 1/4
	30	10 x 1 1/4		40	12 x 1 1/2
	35	10 x 1 3/8		50	12 x 2
	40	10 x 1 1/2		60	12 x 2 3/8
	45	10 x 1 3/4		70	12 x 2 3/4
	50	10 x 2		75	12 x 3
	60	10 x 2 3/8		80	12 x 3 1/4
	70	10 x 2 3/4		90	12 x 3 1/2
	75	10 x 3		100	12 x 4
	80	10 x 3 1/4		110	12 x 4 3/8
	90	10 x 3 1/2		130	12 x 5 1/8
	100	10 x 4		150	12 x 6

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