How Many Door and Window Sensors Do You Need?
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Sensor count is where security-system pricing actually gets decided - and where both sales directions mislead. Provider package builders err generous (a sensor on every window sells more sensors), while starter kits err stingy (a low sticker price wins the comparison table, then you discover "8-piece kit" means four sensors for a house with nine openings). The honest answer comes from a simple principle:
Contact-sensor every door a person can walk through. Then cover ground-level windows by zone - with contacts on the reachable, openable ones, and motion sensors doing the bulk of the interior work.
Widely cited industry figures (treat them as approximate - primary sourcing is thin) put front doors at roughly a third of burglary entries, with first-floor windows and back doors each around a fifth. The pattern that matters for planning: entries overwhelmingly happen at ground level, through ordinary openings, and doors beat windows. Doors first, ground floor second, everything else a distant third.
Room-by-room planning table
| Opening | Recommendation | Why / notes |
|---|---|---|
| Exterior doors (front, back, side) | Contact sensor - always, no exceptions | The most-used entry points, and door sensors are the cheapest, least false-alarm-prone devices in the whole system |
| Door from attached garage into house | Contact sensor | Garage vehicle doors are often left open or defeated; treat the house door as an exterior door. A tilt/contact sensor on the overhead door itself is a nice-to-have, not a must |
| Sliding glass doors | Contact sensor; glass-break optional | A person-sized opening on the private side of the house. Add a rod/track lock too - a $10 mechanical upgrade worth more than a second sensor |
| Ground-floor windows that open, hidden from the street (rear/side) | Contact sensor | Concealment is what burglars want; these are the highest-value window sensors |
| Ground-floor windows facing the street | Contact sensor if budget allows; otherwise motion coverage inside | Visible openings are less attractive; a motion sensor in the room behind them catches entry anyway |
| Basement windows and basement door | Contact sensors, or one motion sensor covering the basement | Classic concealed entry; a single motion sensor often covers several small windows more cheaply |
| Upper-floor windows, no easy access | Skip | Money better spent elsewhere |
| Upper windows reachable from a porch roof, deck, fence, or ladder left outside | Contact sensor (and store the ladder) | "Second floor" only protects you if it's actually hard to reach |
| Interior hallways / stair landings / main living area | 1–2 motion sensors | The backstop: whatever opening you didn't sensor, an intruder still has to cross the house. Pet-immune models handle most cats/dogs under placement guidelines - read your brand's specs |
| Rooms with large fixed panes or many windows | Glass-break sensor (optional) | One acoustic unit covers multiple windows in the same room; catches the smash-and-enter case that contacts miss. Costs more per unit and can false on loud crashes - a tradeoff, not a default |
Two placement notes that prevent the most common DIY mistakes: contact sensor halves must sit within the manufacturer's gap tolerance (roughly half an inch to an inch - check your brand) and adhesive struggles on textured or metal surfaces, so plan screws or extra adhesive there. And motion sensors shouldn't face big windows or heat sources, which drive false alarms - relevant because repeat false dispatches cost real money in many cities (see fines and permit rules).
Worked examples
One-bedroom apartment (upper floor)
- 1 contact sensor - entry door
- 1 contact sensor - balcony slider if the balcony is reachable; otherwise skip
- 1 motion sensor - main living area
Total: 2–3 devices. Most "8-piece" starter kits actually overshoot an apartment; a small kit plus renter-friendly adhesive mounting is plenty. (Renter specifics are covered in our choosing guide.)
Three-bedroom, two-story house (attached garage, slider to patio)
- 3 contact sensors - front door, back/patio slider, garage-to-house door
- 3–4 contact sensors - rear and side ground-floor windows that open and aren't visible from the street
- 2 motion sensors - main floor living space, stair landing or hallway
- Optional: 1 glass-break for a rear room with large panes
Total: 8–10 devices. Notice the mismatch with typical packages in both directions: an "8-piece kit" (which includes non-sensor pieces like the base station, keypad, and range extender) is short, while a salesperson's window-by-window quote of 15+ contacts is padded. The published contents of every major kit are below, and they confirm the shortfall precisely.
Large house (4+ bedrooms, walk-out basement, many ground-floor windows)
- 4–5 contact sensors - all exterior doors including basement walk-out and garage-to-house
- 5–7 contact sensors - concealed/reachable ground-floor and basement windows that open
- 3 motion sensors - main living area, basement, upstairs hallway
- 1–2 glass-breaks - rooms with large panes or French doors, if desired
Total: 13–17 devices. Past this point, each extra contact sensor buys less marginal protection than the motion sensors already provide - a defensible stopping rule when a quote keeps climbing.
What the pre-built kits actually contain
The plan above tells you how many sensors your home needs. The box tells you how many you get. Those two numbers are further apart than the advertised piece count suggests, and you do not have to take our word for the gap: both of the largest DIY brands publish their exact kit contents on their own support pages. We read them on 13 August 2026 and put them side by side.
Ring, from Ring's own tech-spec table
| Kit (advertised piece count) | Contact sensors | Motion detectors | What the rest of the count is |
|---|---|---|---|
| Small Kit (5 pieces) | 1 | 1 | Base Station, Keypad, Range Extender |
| Medium Kit (7 pieces) | 2 | 2 | Base Station, Keypad, Range Extender |
| Large Kit (10 pieces) | 4 | 3 | Base Station, Keypad, Range Extender |
The piece count is honest arithmetic, but it is not sensor arithmetic. In the 5-piece kit, three of the five pieces are infrastructure and exactly one is a door sensor.
SimpliSafe, from SimpliSafe's own package article
| Package | Entry sensors | Motion sensors | What the higher price buys instead |
|---|---|---|---|
| The Foundation | 1 | 1 | Base Station and Keypad only |
| The Starter (SimpliSafe's "Most Popular") | 3 | 1 | Base Station and Keypad only |
| The Hearth | 3 | 1 | Key fob, extra siren, smoke & CO detector |
| The Knox ("for large homes... four or more bedrooms") | 4 | 2 | Key fob, extra siren, smoke & CO detector |
| The Haven | 4 | 2 | Key fob, extra siren, smoke & CO, panic button, water sensor, temperature sensor |
| The Lighthouse | 3 | 1 | 1 outdoor camera |
| The Beacon | 4 | 2 | 2 outdoor cameras |
The pattern nobody selling a kit will point out
Across all ten published kits from the two biggest DIY brands, not one contains more than four door or window sensors. The ceiling holds at the top of both ranges. SimpliSafe's own description of The Knox reads "For large homes, look no further" and calls it "a great fit for homes with four or more bedrooms" - and it ships with four entry sensors, against the nine to twelve our large-house example above actually calls for.
The second pattern matters just as much for budgeting: moving up the range buys different categories of protection, not more perimeter coverage. Across SimpliSafe's entire seven-package ladder the entry-sensor count moves only from one to four. What the more expensive packages add is outdoor cameras, or smoke and carbon monoxide detection, or water and temperature sensors. If you reason "large house, so buy the large package", you are buying a wider kind of monitoring while your doors and windows stay roughly as covered as they were in the mid-range box.
What each of our three examples actually needs on top
| Your home | Door/window sensors the plan calls for | Most any single kit includes | Add separately |
|---|---|---|---|
| One-bedroom apartment | 1–2 | 4 | None. The smallest kit is the right size; anything larger is padding you paid for |
| Three-bedroom, two-story house | 6–7 | 4 | 2–3 contact sensors |
| Large house, 4+ bedrooms | 9–12 | 4 | 5–8 contact sensors |
None of this makes the kits a bad deal, and we are not going to pretend it does. The base station and keypad are unavoidable purchases, the kit is how you buy them, and per-sensor top-up is the cheapest line in the whole project. The practical instruction is narrower than "avoid the kits": price the finished system rather than the box. Take the sensor count from your own walk-around, subtract what the kit includes, and add the difference at the per-sensor price before you compare two brands on their headline package price. Our five-year cost calculator is built for exactly that kind of total, and if you have not settled on a system type yet, the setup finder comes first.
What one motion sensor actually covers
The plan above leans on motion sensors to cover the windows you decide not to sensor, which only works if you know what one unit sees. Two manufacturers publish figures for their current hardware.
SimpliSafe states that its Motion Sensor Gen 2 "protects an entire room from a single location with a 90º field of view and 35 ft. range". A unit in the corner of a room therefore sweeps a quarter-circle roughly 35 feet deep, which for most main living areas is the whole room from one position. That is the reason the planning table says one or two motion sensors for a house rather than one per room.
Ring publishes three numbers for its Motion Detector that are worth having before you mount anything: install it seven feet off the ground; it "will ignore pets weighing under 50 lbs" when the installation guidelines are followed; and "in larger rooms, multiple Motion Detectors may be needed to ensure that there are no blind spots". Ring also gives the wireless range between the Base Station and a device as up to 250 feet over Z-Wave, which is why a range extender is in the box even in the 5-piece kit.
Two honest qualifications. The 50-pound pet threshold is the figure most often repeated vaguely as "pet friendly", and it is conditional: it depends on the mounting guidance being followed, and a 70-pound dog is outside it whatever the marketing implies. And these are two manufacturers' numbers for their own current models. Older sensors, and other brands, differ. Read the spec sheet for the exact model you are buying rather than assuming the category behaves the same way.
The cost math per added sensor
As of July 2026 (verify current pricing - this category discounts constantly): entry/contact sensors from the major DIY brands generally list around $15–$30 each, with frequent multi-pack and promotional discounts well below that (Cove listed door sensors at $7.50 during one of its recurring sales); motion sensors run roughly $30–$50; glass-break sensors cost more than either, which is why they're an "add where justified" device rather than a default.
Put that against the decision it drives: the difference between under-covering and properly covering the example 3-bed house is three or four extra contact sensors - call it $50–$100 once. Compare that to upgrading a monitoring plan tier, which costs $120+ every year (monitoring costs here). Coverage is the better first dollar. The reverse also holds: a quote with six sensors you don't need is $100–$200 of padding, plus the drip of extra batteries to replace. Battery reality check: contact sensors typically run years on a coin or AA cell, so the maintenance cost of a right-sized system is a few dollars of batteries annually - not a reason to under-buy.
One brand-selection note: per-sensor price differences across brands compound with house size. For an apartment, ignore them; for the 15-sensor house, a $10-per-sensor difference is $150, worth checking in our no-contract system comparison before you commit to an ecosystem - expansion sensors only work with their own brand's system, so you're choosing future sensor prices too. Total-budget context lives in our costs and contracts guide.
Make it concrete: walk your home's exterior once, counting doors and reachable openable windows, then apply the table above. Our home security checklist turns the walk into a printable list - including the non-sensor items (locks, lighting, ladder storage) that cover several "should I sensor this?" edge cases for free.
Already have the sensors up and one of them has stopped reporting? A sensor that reads offline is coverage you counted but no longer have. On SimpliSafe, work through a sensor that is not responding; on Ring, see our Ring Alarm troubleshooting guide.
FAQ
Do I need a sensor on every window?
No. Doors always; reachable, openable, concealed ground-floor windows next; motion sensors as the interior backstop for everything else. Upper windows without realistic access don't repay the hardware.
Are glass-break sensors better than window contact sensors?
Different jobs. Contacts detect opening; acoustic glass-breaks detect shattering and cover a whole room of windows at once, catching the smash-and-climb-through case contacts miss. They cost more and can false-alarm on loud sounds. Contacts plus motion sensors is the right default; glass-breaks are a targeted upgrade for big panes and sliders.
Can motion sensors replace window sensors entirely?
Largely, yes - with two caveats. A motion-only setup can't run in "home/stay" mode while you sleep (you'd trip it yourself), so perimeter contacts are what let the system protect you while you're inside. And motion detection means someone is already in the house; contacts on the likely openings give you the earlier alert. That's why the plan above uses both.
Does the biggest starter kit cover a large house?
No, and both brands publish the proof. Ring's largest advertised kit (10 pieces) contains four contact sensors, and every SimpliSafe package from The Foundation to the top of the range contains between one and four entry sensors - including The Knox, which SimpliSafe itself describes as a fit for homes with four or more bedrooms. A large house typically needs nine to twelve, so budget for five to eight beyond the box. See the published kit contents above.
How much of a room does one motion sensor cover?
SimpliSafe publishes a 90º field of view and a 35 ft range for its current Motion Sensor Gen 2, so a corner-mounted unit covers a quarter-circle about 35 feet deep - most main living areas from one position. Ring recommends mounting seven feet up, says its detector ignores pets under 50 lbs when the guidelines are followed, and warns that larger rooms may need more than one to avoid blind spots. Check the spec sheet for your own model; more detail here.
Sources
- MoneyGeek - US Home Burglary Statistics (entry-point figures; industry-cited) (accessed 2026-07-02)
- SimpliSafe - Entry Sensor (specifications, gap tolerance, battery life) (accessed 2026-07-02)
- SimpliSafe Support - What are the Packages Offered for SimpliSafe Security Systems? (contents of all seven packages; article states "Updated June 24, 2026") (accessed 2026-08-13)
- SimpliSafe - The Foundation package page (equipment list; Motion Sensor 90º field of view and 35 ft range) (accessed 2026-08-13)
- Ring Support - Technical Specifications for Ring Alarm Devices (Z-Wave range, motion-detector mounting height and 50 lb pet threshold, battery life) (accessed 2026-08-13)
- Ring Support - Ring Alarm (2nd Gen) tech-spec table (exact box contents of the 5, 7 and 10-piece kits) (accessed 2026-08-13)
- Ring - Alarm Contact Sensor (2nd Gen) (accessed 2026-07-02)
- Ring - Glass Break Sensor (accessed 2026-07-02)
- Security.org - Cove Cost and Pricing (per-sensor sale pricing) (accessed 2026-07-02)
- SafeWise - SimpliSafe Cost (package sensor counts) (accessed 2026-07-02)
- DoItYourself.com forum - motion vs glass-break false-alarm discussion (representative owner experience) (accessed 2026-07-02)