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555 Timer Calculator

Astable frequency, period, high/low times and duty — or monostable pulse width — for the classic 555 timer from R and C. ln(2)-canonical NE555 timing with device-applicability warnings.

Instant result
Result
—

Enter values to calculate.

Inputs—
Mode—
Formula—
Trust summary CVP VERIFIED · CVP protocol 1.0.0-proposed · Engineering assurance · ideal NE555 timing calculation
Input interpretation
Enter values to calculate.
Result
—
Verified scope
ideal NE555 timing calculation
Assurance
Engineering
Declared partition coverage
PASS · 3/3 declared partitions (astable, monostable, invalid-domain) · Matrix
Numerical scope
Ideal NE555 astable/monostable timing vs independent O2 (same V8, ≤2 ULP). Not a claim of assembled-circuit frequency or pulse accuracy.
Known limitations
  • Core CVP does not include live graph, viewport, or pointer interaction.
Model
555 astable frequency/period/high/low/duty or monostable pulse width.
Scope
Classic NE555 ideal timing thresholds (≈⅓ / ⅔ VCC)
Verification
Engine tested · Source checked · v1.2.0 · CVP VERIFIED · CVP protocol 1.0.0-proposed · Engineering assurance · ideal NE555 timing calculation· View Manifest · CVP overview · Specification
Versions
Calculation 1.2.0 · CVP protocol 1.0.0-proposed · Evidence 2026-09-09.claim-scope
Verification revision
2026-09-09.claim-scope · 20/20 property · digest 0682b927e112
Legacy regression
30/30 tests · Production surface contract 8/8
Reference
O1 model · O2 expected_values · O2 numerical_behavior
Interfaces
PASS · UI (SSR) / REST / MCP — ui-ssr is query-result HTML, not a live browser session.
Supplemental domain review
Internal · Pass · electrical-engineer · 2026-08-13
Named expert review
Not performed
CVP suite
2/2 golden · 7/7 CVP boundary · 5/5 invalid · 20/20 property · 2/2 cross-interface · 8/8 CVP contract · Manifest
Sources
Sources
Evidence
3 legacy golden · 7 legacy boundary · legacy regression suite · 2/2 oracle-backed golden · 5/5 invalid · Artifact integrity PASS · CalculatorX electrical review
This calculator CURRENT · Public schema 1.2.0 matches · Semantic contract ✓ · Production attested · Public/cache ✓ · Origin ✓
Semantic contract
PASS
Full verification

Manifest identity, reference classes, interfaces, suite, and production records.

Formulas

Core equations used by this calculator.

Astable high / low (canonical)tH = ln(2)·(R1+R2)·C · tL = ln(2)·R2·C
Astable period / frequencyT = tH+tL · f = 1/T
Astable dutyduty = (R1+R2)/(R1+2·R2)
Monostable pulse widthtw = 1.1·R·C
Datasheet rounded forms0.693 ≈ ln(2) · 1.44 ≈ 1/ln(2)
iCanonical engine uses ln(2) for astable timing so tH, tL, T, and f stay internally consistent. TI datasheet 0.693 / 1.44 forms are documented rounded aliases (1/ln(2) ≈ 1.442695 ≈ 1.44). Device families differ in timing accuracy, leakage, output/discharge characteristics and usable frequency range; verify the selected device datasheet.

How to use

1

Choose astable or monostable

Astable needs R1, R2, and C for frequency, period, high/low times and duty. Monostable needs R and C for pulse width.

2

Enter resistances and capacitance

Use engineering units (kΩ / MΩ, µF / nF / pF). Values must be greater than zero. Share URLs and the API use SI (Ω, F).

3

Read timing — and heed device warnings

Astable returns f, T, tH, tL and duty from a single ln(2)-based model. Standard wiring duty is always >50%. Frequencies above 100 kHz and monostable pulses below ~10 µs trigger applicability warnings from the NE555 datasheet guidance.

Example calculations

Common configurations with formula and result.

ϟ

Astable

R1=1 kΩ · R2=10 kΩ · C=1 µF

tH=ln(2)(1k+10k)·1µF · T=tH+tL · f=1/T
≈68.70 Hz · duty 52.38% · T≈14.556 ms
ϟ

Monostable

R=10 kΩ · C=1 µF

1.1·10k·1µF
tw = 11 ms

555 Timer calculator specification

Version 1.2.0 · Engine tested · Supplemental domain review · Internal · 2026-08-13

Calculation status

Review policy · Evidence · Reviewed by CalculatorX electrical review (electrical-engineer)

Definition
Astable (canonical): tH = ln(2)·(R1+R2)·C, tL = ln(2)·R2·C, T = tH+tL, f = 1/T, duty = (R1+R2)/(R1+2·R2). Monostable: tw ≈ 1.1·R·C. TI datasheet 0.693 / 1.44 are rounded aliases of ln(2) and 1/ln(2).
What it calculates
555 astable frequency/period/high/low/duty or monostable pulse width.
Inputs
  • mode astable|monostable (oneOf)
  • astable: R1,R2,C
  • monostable: R,C
Outputs
  • astable: f_Hz, period_s, high_time_s, low_time_s, duty
  • monostable: pulse_width_s
Formula
tH=ln(2)(R1+R2)C; tL=ln(2)R2C; f=1/T; tw=1.1RC
Assumptions
  • Classic NE555 ideal timing thresholds (≈⅓ / ⅔ VCC)
  • Standard astable topology without duty-cycle diode
  • Astable timing uses ln(2); TI 0.693/1.44 are rounded aliases
Units
  • Ω, F → Hz or s
Boundary conditions
  • R or C ≤0 → VALUE_MUST_BE_POSITIVE
  • Missing mode or mode-required fields → MISSING_REQUIRED_INPUT
  • Bad mode → INVALID_MODE
  • f > 100 kHz → NE555_HIGH_FREQUENCY warning
  • Monostable tw < ~10 µs → MONOSTABLE_SHORT_PULSE warning
Example
R1=1k R2=10k C=1µF → ~68.70 Hz · duty 52.38%
Validation cases

2 published on this page · 30/30 tests · Production surface contract 8/8 · View evidence

  • astable R1=1000 R2=10000 C=1e-6 → f≈68.70 Hz · T≈14.556 ms · duty≈52.38%
  • monostable R=10000 C=1e-6 → tw=11 ms
Sources
  • NE555 datasheet timing equations (TI xx555 Precision Timers) — Astable / monostable
    Supports: tH/tL ≈ 0.693·RC (rounded ln(2)); f≈1.44/((R1+2R2)C) rounded 1/ln(2); tw=1.1RC; ≤100 kHz guidance; ~10 µs monostable floor
  • Horowitz & Hill — The Art of Electronics — 555 timer circuits
    Supports: Astable / monostable RC timing identities
Last reviewed
2026-08-13
Reviewed by
CalculatorX electrical review (electrical-engineer)
Calculation version
1.2.0

Background

Interpretation and common distinctions.

Compute 555 timer astable or monostable timing from a single ln(2)-canonical NE555 model — with full astable timing (f / T / tH / tL / duty) and device-applicability warnings.

Status: Internally reviewed — electrical engineering · pin calculation_version: 1.2.0.

v1.2.0 changelog: Astable timing now uses one ln(2)-based canonical model (tH, tL, T=tH+tL, f=1/T). TI datasheet 0.693 / 1.44 expressions remain documented rounded forms (0.693 ≈ ln(2), 1.44 ≈ 1/ln(2)).

Supported and not supported

Supported — Astable closed forms (frequency, period, high/low, duty) · Monostable pulse width · NE555 high-frequency and short-pulse warnings · API electrical.timer_555

Not supported — Full SPICE · CMOS-specific electrical models · Duty-cycle diode trick (<50%) · Automatic R/C E-series design (future Designer surface)

Standard astable topology

VCC
 │
R1 (RA)
 ├── DISCH
R2 (RB)
 ├── THRES / TRIG
 C
 │
GND

Capacitor charges through R1+R2 and discharges through R2 only — that is why the ideal duty is always > 50%.

Agent / API notes

Capability id: electrical.timer_555 · tool id: timer-555 · pin 1.2.0.

Input Schema is mode-discriminated oneOf:

  • astable — requires R1, R2, C
  • monostable — requires R, C

Canonical result is also mode-discriminated (inputs are echoed separately; result does not repeat R/C).

Errors: MISSING_REQUIRED_INPUT, VALUE_MUST_BE_POSITIVE, INVALID_MODE.

Warnings: NE555_HIGH_FREQUENCY, MONOSTABLE_SHORT_PULSE.

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Frequently asked questions

Key distinctions behind the calculation.

Can duty be below 50%?

With the standard astable wiring (charge via R1+R2, discharge via R2 only), duty is always >50%. Use a diode across R2 for <50% designs (not modeled here).

Why a warning above 100 kHz?

The NE555 datasheet recommends staying at or below about 100 kHz for reduced astable waveform distortion. The ideal formula still computes; the warning means you should check the device datasheet or consider a CMOS timer.

Why don’t results match 1.44/((R1+2R2)C) exactly?

TI publishes both 0.693·RC and f≈1.44/((R1+2R2)C) as rounded shortcuts. 1/0.693 ≠ 1.44 exactly. CalculatorX uses ln(2) as the single canonical constant so tH, tL, T, and f stay consistent; 0.693 and 1.44 remain documented datasheet aliases.