If you lost one key and still have the other, the lost key still opens your lock. Copying the spare only restores your own access. The real question is who else might now get in, and the choice is to leave the lock alone, rekey it, or replace it. Rekeying stops the lost key from working while keeping the lock. Replacing fits a worn lock or a change of lock type.
Why the spare doesn't settle it
A lost key is a working key you no longer control. The spare in your hand proves you can still get in. It says nothing about who else can. Until the lock changes, the missing key turns the lock just as yours does.
Copying the spare solves a separate problem: being down to one key and one slip away from a lockout. That is worth fixing. The lost key keeps working after the copy is cut, though.
Rekeying: the same lock answers to a new key
Rekeying changes the cylinder, the part the key slides into. Inside a pin-tumbler cylinder sit small spring-loaded pins of different lengths. The lock turns only when a key's cuts lift each pin to the right height. Rekeying swaps those pins for a new set matched to a new key pattern, so the old keys no longer line them up. That includes the lost one.
The lock body, the bolt, and the trim on the door stay where they are. Your remaining key stops working too, and you get keys cut to the new pattern.
Rekeying suits a lock that is sound and fits the door well. If the key turns smoothly and the bolt extends fully, rekeying takes the lost key out of the picture without changing anything you are happy with.
Replacing: when the lock itself needs to go
Replacing the lock also retires the lost key, since a new lock comes with its own keys. What replacement adds is new hardware. That only matters if the old hardware is falling short.
You can spot a lock falling short without taking anything apart. The key sticks or needs jiggling. The bolt drags or stops short of fully extending. The lock or handle wobbles on the door. Rekeying a lock like that gives you new keys in a worn mechanism.
Replacement is also the route to a different kind of lock. A high-security lock pairs a cylinder built to resist manipulation with restricted keys, meaning the key blanks are controlled so copies are harder to get. A smart lock or keypad lets you change codes instead of chasing keys. If the lock works well and you only want the lost key to stop working, replacement is more than the situation calls for.
How urgent is it to change the lock after losing a key?
Start by confirming the key is really gone. Check coat pockets, bags, and wherever things land by the door, and ask anyone who shares the space whether they picked it up.
If the key is truly missing, these factors set the urgency:
- Where it was lost. A key dropped far from home is less useful to a finder than one lost near your own door.
- Whether it points to the address. A tag with your address, a fob with a company name, or a bag that also held ID ties the key to the lock.
- Who could have it. A busy public place means unknown finders. A key a former roommate or employee never returned has a known holder.
- How the lock behaves. Sticking or looseness shifts the choice from rekeying toward replacing.
- Whether others share the door. In a business or managed building, one unaccounted key leaves the key record incomplete, and rekeying gives everyone a clean starting set.
Leaving the lock alone is reasonable in a narrow case: the key was lost far from the property, carried nothing linking it to you, and nobody else shares the door. Even then, copy the spare so you are not down to a single key.
What to have ready before you call
If you decide to rekey or replace, a few details help the conversation along: which door the lock is on, how many keys you still hold, and how the lock behaves now, smooth or sticking. For a shared or commercial door, a rough count of who should end up with keys helps too.
Long Island City Locksmith handles lock change and rekey work, high-security locks, and smart locks and keypads, and offers mobile locksmith service. Call (917) 500-5130 to talk through which option fits the lock you have.
Frequently asked questions
Should I copy my spare key before deciding whether to rekey?
If you plan to rekey or replace the lock soon, hold off on copying the spare. Any copy cut from your current key stops working along with the lost one once the lock changes. Copying first makes sense if you decide to leave the lock as it is, or if relying on one key for a while worries you more than a wasted copy.
Does rekeying mean I get new keys for everyone who uses the door?
Yes. Rekeying a lock changes the key pattern it accepts, so all existing keys stop working, including the one you still hold. Each person who should have access needs a key cut to the new pattern. That makes rekeying a natural moment to decide who actually needs a key and hand out only that many.
If I lost the key at home and might find it, how long is it reasonable to wait?
A key that never left the home and carried nothing identifying is low risk while you search, because no stranger has it. Once you have checked the likely spots and asked everyone in the household, waiting longer gains little. If the key might have left the home on a ring with your address, treat it as lost outside and do not wait on the search.
Can a property manager rekey one unit without changing the whole building's locks?
Generally, yes. Each lock cylinder is rekeyed on its own, so one unit's lock can take a new key pattern while the rest of the building stays as it is. The catch is a master key system, where one key opens many locks. There, the rekeyed cylinder still has to accept the building's master key, which takes planning around the existing key record.