Buying a safe by eyeballing it is how people end up with a $2,000 unit that can't fit a filing box, or a floor safe with 100 litres of dead space they paid for and never use. Calculating safe capacity properly takes fifteen minutes with a tape measure and a calculator — this guide walks through the exact method.
- Convert every item's dimensions to litres using length x width x height / 1000, then sum the total.
- Add a 30-50% buffer to your total litre figure — safes lose usable space to shelving, locking bolts and door thickness.
- The Chubb Mini Banker MkII range (M-15 to M-225) names its own litre capacity in the model number, making it the easiest starting benchmark in 2026.
- Document safes need extra allowance for folders and lever-arch files, which take up more volume than flat paper.
- Buy the next size up when your calculated total lands within 10% of a safe's rated capacity — running a safe at 100% full damages the locking mechanism over time.
Why this matters
Most people size a safe by the outside dimensions on a listing page, not the usable interior. That's backwards. A safe's steel walls, fire-lining and locking bolt housing can eat 15-25% of the external cabinet before you've stored a single item, and shelving brackets take another slice.
Getting the calculation wrong in either direction costs money. Undersize it and you're buying a second safes within two years. Oversize it and you've paid for litres you'll never fill — cash and document safes are usually priced per litre of rated capacity, so every unused litre is wasted spend.
What you'll need
- A tape measure (metric, at least 1 metre)
- A written inventory of everything going in the safe — documents, jewellery boxes, cash, firearms, hard drives
- A calculator or spreadsheet
- The interior dimensions of any safe model you're comparing (listed on most product pages)
- 10 minutes to measure irregular items like jewellery boxes or lever-arch folders
The steps
1. List every item, not just the big ones
Write down every single item that needs to go in the safe, including things you plan to add in the next 12 months — passports, a spare set of car keys, a watch box you're expecting for Christmas. This accomplishes the single most common failure point in safe buying: people size for today and outgrow the unit by the following year.
Common mistake: forgetting bulky non-valuables like external hard drives or a laptop, which take up disproportionate volume relative to their value.
2. Measure irregular items in centimetres
For anything that isn't a flat stack of paper — jewellery boxes, watch cases, ammunition boxes, hard drive enclosures — measure length, width and height in centimetres. Round up to the nearest centimetre; underestimating here is the fastest way to end up with a safe that's 2cm too shallow for a document folder.
A standard A4 lever-arch folder measures roughly 32cm x 29cm x 8cm. A mid-size jewellery box averages 25cm x 20cm x 10cm. Use these as reference points if you don't have the item in hand yet.
3. Convert dimensions to litres
Multiply length x width x height in centimetres, then divide by 1,000 to get litres. A 32cm x 29cm x 8cm lever-arch folder works out to roughly 7.4 litres. Do this for every item on your list and add the totals.
Expected outcome: a single number, in litres, representing your raw storage need before any buffer is applied.
Common mistake: measuring a stack of loose paper and assuming it behaves like a solid block — loose documents in a safe need folders or boxes around them, which adds 20-30% more volume than the paper itself.
4. Add a 30-50% buffer
Take your raw litre total and multiply by 1.3 to 1.5. This accounts for shelving thickness, the space lost to a locking bolt housing along one wall, and the practical reality that nobody packs a safe to the exact millimetre. Cash and document safes on the lower end of that range (30%); firearms and irregular-shaped items need the higher end (50%) because rifles and ammo boxes don't stack efficiently.
5. Match your total against rated capacity
This is where a named litre rating makes life easier. The Chubb Mini Banker MkII Size 3 (M-40) is rated at 40 litres — its own model number tells you the capacity. Compare your buffered total against a range of rated capacities and pick the size that sits at 70-85% of maximum, not right at the ceiling.
Common mistake: buying a safe that matches your current total exactly. That leaves zero room for growth and forces a second purchase within 18-24 months.
6. Check interior layout, not just volume
Two safes can both be rated at 75 litres and behave completely differently — one with a single deep cavity, another split across three shelves. Shelved interiors are better for document folders and jewellery boxes that need to sit flat; open cavities suit cash bags and irregular items. The Yale Document Fire Safe Extra Large (YFM/520/FG2) is built with document storage specifically in mind, so its layout favours flat A4 filing over bulky boxes.
7. Confirm the external footprint fits the install space
A safe that clears your litre calculation can still fail on external dimensions — too deep for a wardrobe cavity, too tall for a cupboard shelf. Measure the intended install location last, after you know which capacity range you need, so you're not compromising on capacity to force a fit.
Troubleshooting
Total came out lower than expected, but items still don't fit. You likely measured items loose rather than in their storage containers — jewellery in boxes, documents in folders. Re-measure with the actual container, not the item alone.
Safe is rated bigger than my total but feels cramped. Rated litre capacity is gross interior volume, not usable volume after shelves are installed. Subtract 10-15% for any safe with fixed internal shelving.
Firearms don't fit even though the litre maths checks out. Long arms need vertical clearance, not just volume — a 100-litre safe with a 90cm interior height won't fit a rifle with a scope attached, regardless of total litres.
Documents keep curling or jamming against the door. The safe's depth is likely shorter than a standard A4 folder plus clearance. Interior depth needs to be at least 2cm longer than your longest folder.
Cash safe feels full at half its rated capacity. Loose banknotes and coin bags don't pack efficiently — treat cash storage litre needs at double the raw volume of the notes themselves.
Tools and resources
- Tape measure and calculator (the only physical tools required)
- A written item inventory, updated annually
- Product pages with listed interior dimensions — check before buying, not after
- The CMI G-CC2 Class C filing cabinet for anyone whose calculation lands mostly in flat A4 document storage rather than mixed valuables
- A second measurement pass on install day, before the safe is delivered, to catch any gap between plan and reality
What to do next
Once you've got a litre total, the next decision is external footprint versus room layout — that's a separate calculation from capacity itself. The guide on how to pick the right safe size for your home walks through matching your buffered capacity number against realistic install spaces in an Australian home.
FAQ
How do I calculate safe capacity in litres?
Multiply length x width x height in centimetres for each item, divide by 1,000, then add all the totals together. Add a 30-50% buffer on top to account for shelving and locking hardware before matching the figure to a rated safe capacity.
What does a safe's litre rating actually mean?
It's the gross interior volume, not the usable space after shelves are fitted. Rated capacity for models like the Chubb Mini Banker MkII is printed directly in the model number, such as M-40 for 40 litres.
How much buffer should I add for firearms storage?
Use the higher end of the 30-50% range, closer to 50%, because rifles and ammunition boxes don't pack as efficiently as flat documents or cash. Vertical clearance matters as much as raw litres for long arms.
Is a bigger safe always better value?
No — cash and document safes are typically priced per litre of rated capacity, so buying well beyond your calculated need with 50% buffer wastes money. Aim for 70-85% fill at rated capacity, not maximum headroom.
How much space do document folders take up in a safe?
A standard A4 lever-arch folder measures roughly 32cm x 29cm x 8cm, which works out to about 7.4 litres. Loose paper in folders takes 20-30% more volume than the same paper stacked flat.
Do jewellery boxes need extra buffer room?
Yes, irregular-shaped items like jewellery boxes and watch cases pack less efficiently than flat stacks, so treat them at the higher end of the 30-50% buffer range.
What's the difference between external and internal safe dimensions?
External dimensions are what fits your install space; internal dimensions and rated litre capacity are what determines what actually fits inside. Steel walls and fire-lining can consume 15-25% of the external cabinet.
Should I size a safe for today's items or future growth?
Size for growth — a safe filled to exactly today's calculated total leaves no room and often forces a second purchase within 18-24 months. Add items you expect to acquire in the next year to your inventory before calculating.
One last thing
The Chubb Mini Banker MkII range runs from the M-15 at 15 litres right up to the M-225 at 225 litres — a 15x jump across seven sizes in the same product line. That spread exists because most buyers get the capacity calculation wrong in one direction or the other, and the manufacturer would rather offer granular steps than have you guess. Do the litre maths first in 2026, then pick the model number that matches it.