Screws & bolts · free toolkit
Which drill bit, which torque, which wall plug? Ask here.
Seven small calculators with the standard tables built in. Pick your question below, set two or three things, read the answer in one sentence.
- Free, no limit
- No account, no upload
- Metric and inch
- Printable charts
What do you want to know?
- Which drill bit for this screw?Pilot hole, clearance hole, countersink and screw length
- Tap drill for a threadM1.6 to M64, UNC and UNF, with clearance holes
- How tight is this bolt?Torque and preload for 8.8, 10.9, 12.9, grade 5 and 8
- What thread is this?Measure diameter and pitch, get the standard name
- Which wall plug for this wall?Plug, drill bit and screw for six wall types
- How many screws do I need?Drywall and decking, with spare included
- Convert and printNm to ft·lb, mm to inch, TPI, gauge, and the charts
Which drill bit for this screw?
Three holes make a clean joint: a pilot hole for the thread, a wider clearance hole through the top piece so the boards pull together, and a countersink for the head.
Why these sizes, and tips for the material
The pilot hole is roughly the root of the screw (the shank without the thread): about 70 percent of it in softwood, about 90 percent in hardwood and MDF. The clearance hole is the outer thread diameter plus a little, so the top piece is pulled down by the head instead of being lifted by the thread.
Tap drill and clearance hole
The hole you drill before cutting a thread with a tap, and the through-hole for a bolt that just passes through.
Thread details and notes
- Minor diameter
- —
- Pitch
- —
- Tensile stress area
- —
How tight is this bolt?
Torque is only the way to reach a clamping force. Most of it goes into friction, so an oiled bolt at the same torque is much tighter than a dry one.
!Same bolt, other conditions
| Condition | Torque | Clamping force |
|---|
How this is calculated
- Tensile stress area
- —
What thread is this?
Two measurements are enough: the outside diameter and the distance between two thread crests. The calculator matches metric coarse and fine, UNC, UNF and Whitworth.
Which wall plug for this wall?
The wall decides the type of plug, the weight decides the size. Tap the wall: a dull thud is solid, a hollow ring is plasterboard or hollow brick.
Shopping list per fixing
How to drill it, and the safety note
Loads are per fixing with a margin for household use. Where people could be hurt if it fails (railings, awnings, anything above a bed or cot), use an anchor with a building approval and its published load table.
How many screws do I need?
Drywall by sheet and stud spacing, decking by board and joist spacing. Ten percent spare is included, because one always snaps.
Shopping list
Which screw and why this number
Convert units and print the charts
Torque, length, pitch and screw gauge conversions, plus the three tables people tape inside the workshop cupboard.
Metric tap drill and clearance holes (ISO 261 / ISO 273)
| Thread | Pitch mm | Tap drill mm | Clearance fine | Medium | Coarse |
|---|
Unified tap drills (UNC / UNF)
| Size | Major in | UNC TPI | UNC tap drill | UNF TPI | UNF tap drill | Clearance |
|---|
Wood screw pilot holes (mm)
| Screw | Softwood | Hardwood / MDF | Clearance | Countersink |
|---|
Need the screws, plugs or drill bits for the job?
The sizes above follow DIN, ISO and ASME standards, so any well-stocked supplier carries them. For metric screws, wall plugs and drills in small and bulk quantities, Schraubenhandel24 (German shop, ships across Europe) stocks the full range by diameter, length and strength class. Copy the answer above and paste it into their search.
Where the numbers come from
Nothing on this page is guessed. Pilot holes come from the root diameters of common wood and chipboard screws and the usual rule that softwood needs a hole around 70 percent and hardwood around 90 percent of that root. Tap drills follow the DIN and ISO tables, which are the "diameter minus pitch" rule rounded to stock drill sizes; the Unified table uses the number and letter drills every US chart lists. Clearance holes are the fine, medium and coarse series of ISO 273 and the close and normal fits of ASME B18.2.8.
Torque
Metric torque is computed to VDI 2230: the clamping force is the chosen share of the proof strength times the tensile stress area, reduced for the torsional stress the thread friction adds, and the torque is that force times the sum of the pitch term, the thread friction term and the head friction term. Inch torque uses the SAE method, torque = K × clamp load × diameter, because that is how every published grade 5 and grade 8 chart was made. Both share one truth: change the lubrication and the same torque gives a very different clamping force. Always state the condition with the number.
Wall plugs
The anchor table encodes what plug manufacturers publish for household fixings: nylon expansion plugs in solid materials, knotting universal plugs for hollow brick, dedicated plugs for aerated concrete, cavity anchors or the studs for plasterboard. The load classes carry a margin for typical use. They are not a substitute for an approved anchor and its load table where a failure could hurt someone.
What this page does not do
It does not size a bolted joint; preload, embedding loss, external load and fatigue are a job for an engineer with the real geometry. It also does not know your specific screw: a manufacturer's pilot hole recommendation for a particular screw type beats the generic table, so use it when you have it.
Questions people ask before they drill
What size pilot hole do I drill for a wood screw?
Roughly the root diameter of the screw, the part without the thread: for a 4 mm screw 2.5 mm in softwood and 3 mm in hardwood, for a #8 screw 3/32 inch in softwood and 7/64 inch in hardwood. The top piece gets a clearance hole the size of the outer thread so the boards pull together instead of jacking apart.
What is the tap drill size for M6, M8 and M10?
M6: 5.0 mm, M8: 6.8 mm, M10: 8.5 mm. The rule is nominal diameter minus pitch, which gives about 75 percent thread engagement, the sweet spot between thread strength and tapping effort. The tap drill tab has the full table from M1.6 to M64 and for UNC/UNF.
How much torque for an M10 8.8 bolt?
Around 45 to 55 Nm dry and zinc-plated, depending on the friction assumed, for a clamping force of about 26 to 30 kN. A lubricated bolt needs noticeably less torque for the same force, which is why over-tightening happens so often after someone oils a thread "to make it easier".
Which wall plug do I need for a TV bracket?
In concrete or solid brick a 10 mm nylon plug or frame anchor with 7 to 8 mm screws; in hollow brick an injection anchor with a mesh sleeve; in plasterboard fix into the studs or use metal cavity anchors. The wall plug tab gives plug, drill and screw size for six wall types and four load classes.
Can I identify a bolt without a thread gauge?
Yes, with a ruler: count the crests over 10 mm for metric or over one inch for Unified, and measure the outside diameter with a calliper. Enter both in the identifier and it lists the closest standard threads with a confidence rating and how to tell near-identical candidates apart.
Is the screw calculator free?
Yes. Everything on this page runs in your browser. There is no account, no daily limit and nothing is uploaded.