3mm Round Top LED

3mm round-top LEDs (T-1 package) are the smallest standard through-hole LED, measuring just 3mm across the lens.

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3mm round-top LEDs (T-1 package) are the smallest standard through-hole LED, measuring just 3mm across the lens. That compact footprint makes them the default choice for model railroading in HO and N scale, guitar pedal indicator lights, miniature wargaming terrain, slot car headlights, and any project where a 5mm LED would look oversized or physically will not fit. We stock 3mm clear round-top LEDs in 9+ colors including red, orange, amber, yellow, green, blue, white (cool and warm), UV, and pink — each product page lists the exact peak wavelength in nanometers so you can color-match across a build.

Model railroad applications: 3mm LEDs are the standard replacement for grain-of-wheat incandescent bulbs in HO and N scale locomotives from Atlas, Kato, Walthers, Athearn, and Broadway Limited. Cool white produces the crisp headlight beam of modern diesel units; warm white recreates the softer glow of steam-era locos and passenger cars. Red 3mm LEDs serve as end-of-train markers and classification lights. Green and amber work for block signals and crossing flashers. The 3mm body fits inside locomotive headlight housings and scale signal heads without modification — a 5mm LED typically requires trimming the housing or the LED itself. Model railroaders building N scale structures use 3mm warm white LEDs for building interiors, porch lights, and station platform lamps, where anything larger would be out of scale.

Guitar pedals, wargaming, and beyond: DIY stompbox builders on forums like DIYPB, freestompboxes.org, and r/diypedals use 3mm round-tops when the enclosure bezel only accepts a 3mm LED holder, or when the pedal face is too crowded for a 5mm indicator. Red and blue are the standard bypass indicator colors; green signals buffered or active status. Warhammer 40K painters and miniature wargaming hobbyists wire 3mm LEDs into Dreadnought eye sockets, Land Raider headlights, Daemon Prince weapon effects, and terrain scatter for Object Source Lighting (OSL). The tiny body hides inside a 28mm scale model where even a 5mm LED would protrude. Slot car racers wire 3mm white LEDs into 1:32 and 1:24 scale bodies for headlights, and red for taillights — the T-1 package is proportionally correct at these scales.

Colors and lens characteristics: All 3mm round-top LEDs in this category have a water-clear lens that focuses light into a narrow 15–30° forward beam. This produces the highest on-axis brightness (measured in millicandela, mcd) of any 3mm package — ideal for headlights, signal beams, and fiber-optic coupling. If you need wider-angle visibility for panel indicators or building interiors, consider our diffused 3mm LEDs, which have a frosted lens spreading light across 120–160°. White LEDs come in two color temperatures: warm white (~3000–4000K) for incandescent-like warmth, and cool white (~5500–6500K) for a crisp, daylight appearance. UV LEDs (395–405nm) are useful for curing UV resin in miniature basing and activating fluorescent paint in diorama and haunted house builds.

Electrical specs and resistor selection: Forward voltage by color: red/orange/yellow/amber ≈ 2.0–2.2V; blue/green/white/UV ≈ 3.0–3.2V. Maximum forward current: 20mA. Every bare LED requires a series current-limiting resistor — without one, the LED will overdraw current and burn out in seconds. Common resistor values: at 12V, use 510Ω for red or 470Ω for white/blue; at 5V (Arduino), use 150Ω for red or 100Ω for white. Use our LED resistor calculator for any supply voltage. For AC power sources — model railroad transformers, DCC track power, or 12V landscape lighting wire — add a bridge rectifier and smoothing capacitor to convert to DC before the LED circuit. See the AC/DCC wiring guide for a complete diagram. New to LEDs? Pre-wired LEDs are the easiest starting point — the resistor is already built in, so you just connect power and ground.

Mounting and physical dimensions: The 3mm LED body fits a 3.2mm (1/8″) panel-mount hole for a snug press-fit. For a professional finish, use a 3mm LED holder or bezel — the LED snaps in from the front and locks from the back. Lead spacing is standard 2.54mm (0.1″), compatible with solderless breadboards, perfboard, and standard-pitch PCBs. The longer lead is the anode (+); the shorter lead with a flat spot on the base is the cathode (−). For two slot car headlights running on 12V track power, wire two white 3mm LEDs in series with a 220–330Ω resistor to split the voltage drop efficiently.

Related categories: If 3mm is too small for your application, 5mm round-top LEDs are the next size up and the most popular LED size overall. For larger displays, see 10mm round-top LEDs. Need automatic blinking or color-cycling? Browse animated LEDs with a built-in controller IC — no microcontroller needed. For surface-mount projects, SMD LEDs go as small as 0402 package. Looking to couple light into a strand? Pair a clear 3mm LED with our fiber optic products for pinpoint lighting effects in dioramas and architectural models.

Frequently Asked Questions

Forward voltage (Vf) depends on the LED color. Red, orange, amber, and yellow LEDs typically need 2.0–2.2V. Blue, green, white, UV, and RGB LEDs typically need 3.0–3.2V. The exact Vf for each product is listed on its product page. Always design your resistor circuit around Vf — if your supply voltage is below Vf, the LED will not light at all.
Use our LED Resistor Calculator to find the exact value for your supply voltage and LED color. At 12V: red ≈ 510Ω, white/blue ≈ 470Ω. At 5V (Arduino): red ≈ 150Ω, white/blue ≈ 100Ω. At 3.3V: red ≈ 68Ω. Each product page includes a quick-reference resistor table for common voltages, or use our LED resistor calculator for any supply voltage. New to LEDs? Pre-wired LEDs include the resistor on the wire — no math required.
Use a 3.2mm or 1/8″ drill bit for a snug press-fit. For panel-mount applications where you want a clean, professional finish, use a 3mm LED holder or bezel — the LED snaps into the holder from the front and locks from the back with a retaining ring. Holders are available in chrome and black finishes.
The number is the lens diameter. 3mm LEDs (T-1 package) fit tighter spaces and look proportionally correct in HO and N scale models, slot cars, and miniature wargaming terrain. 5mm LEDs (T-1¾ package) are brighter at the same current, easier to handle, and have the widest selection of colors and styles. Electrically they are identical — same forward voltage ranges by color, same 20mA max current, same wiring and resistor formulas. Choose based on the physical space available and the scale of your project.
LEDs are DC-only devices. On raw AC, they flicker at line frequency and the reverse voltage can damage them over time. For AC model railroad transformers or DCC track power, add a bridge rectifier and a smoothing capacitor to convert to clean DC. The same applies to 12V AC landscape lighting wire. See our AC/DCC wiring guide for a complete wiring diagram with component values.
Yes — a clear-lens cool white 3mm LED at 20mA produces thousands of mcd, far brighter than the grain-of-wheat incandescent bulbs they replace in Atlas, Kato, and Athearn locomotives. At HO and N scale, the focused beam of a clear round-top 3mm LED creates a realistic headlight effect on the layout surface. For building interiors where you want softer, more diffused light, use a diffused 3mm LED or a warm white color temperature.