RTC Clock Accuracy Utility

RTC Clock Drift Calculator

Calculate real-time clock drift from oscillator error in parts per million. Find seconds gained or lost per day, month and year, actual oscillator frequency, frequency error, drift over a custom interval and required calibration correction.

✓ PPM Drift ✓ Seconds / Day ✓ Monthly Drift ✓ Annual Drift ✓ 32.768 kHz RTC ✓ Calibration

RTC

Clock Drift & Calibration

● Ready

RTC Frequency Error (ppm) Positive ppm means the clock runs fast; negative ppm means it runs slow.

Nominal RTC Frequency (Hz) 32.768 kHz is common for watch crystals and low-power RTC oscillators.

Custom Time Interval Enter the elapsed reference time over which you want to calculate drift.

Custom Interval Unit The custom drift output uses this interval.

Observed Clock Drift (optional, seconds) Optional: enter measured gain/loss to calculate measured ppm. Positive = clock gained time.

Observed Test Duration (optional, hours) Used together with observed drift to calculate the measured oscillator error.

Core formula: drift seconds = elapsed seconds × ppm / 1,000,000. Therefore a +20 ppm RTC gains approximately 86,400 × 20 / 1,000,000 = 1.728 seconds per day. The sign is preserved: positive means fast/gaining; negative means slow/losing.

RTC Clock Drift Result Calculated

RTC Accuracy Summary —

Drift Per Day —

Drift Per Week —

Drift Per 30 Days —

Drift Per Year —

Custom Interval Drift —

Nominal Frequency —

Actual Frequency —

Frequency Error —

Clock Direction —

Required Correction —

Measured PPM —

Measured Correction —

Time to Drift 1 Second —

Time to Drift 1 Minute —

Relative Error —

Accuracy —

Calculation Breakdown

—

What Is RTC Clock Drift?

RTC clock drift is the difference that accumulates between a real-time clock and an accurate reference clock. Even a small oscillator frequency error can produce noticeable time error after days, months or years of continuous operation.

RTC accuracy is frequently expressed in parts per million, or ppm, because the required frequency errors are very small compared with the oscillator’s nominal frequency.

RTC PPM Drift Formula

Drift Seconds =

Elapsed Seconds × PPM
---------------------
1,000,000

The formula preserves the sign of the ppm value. A positive value describes a fast RTC that gains time, while a negative value describes a slow RTC that loses time.

How Much Is 1 PPM Per Day?

One day contains 86,400 seconds. A frequency error of one part per million therefore produces:

86,400 × 1
------------
1,000,000

= 0.0864 seconds/day

That is approximately 86.4 milliseconds of time drift per day.

RTC Drift at Common PPM Values

Error	Per Day	Per 30 Days	Per 365 Days
1 ppm	0.0864 s	2.592 s	31.536 s
5 ppm	0.432 s	12.96 s	157.68 s
10 ppm	0.864 s	25.92 s	315.36 s
20 ppm	1.728 s	51.84 s	630.72 s
50 ppm	4.32 s	129.6 s	1576.8 s

20 PPM RTC Drift Example

RTC Error:
+20 ppm

Daily drift:

86,400 × 20 / 1,000,000
= 1.728 seconds

30-day drift:

2,592,000 × 20 / 1,000,000
= 51.84 seconds

365-day drift:

31,536,000 × 20 / 1,000,000
= 630.72 seconds

≈ 10 minutes 30.72 seconds

What Does Positive PPM Mean?

A positive frequency error means the oscillator frequency is above its nominal value. The RTC therefore advances slightly faster than real time and gains time.

Nominal:
32,768 Hz

Error:
+20 ppm

Actual frequency:
32,768 × (1 + 20 / 1,000,000)

What Does Negative PPM Mean?

A negative ppm value means the oscillator runs below the nominal frequency. The RTC therefore advances too slowly and loses time relative to the reference.

Error:
-20 ppm

Daily drift:
-1.728 seconds

Meaning:
RTC loses 1.728 seconds per day

RTC Frequency Error Formula

Frequency Error =

Nominal Frequency × PPM
-----------------------
1,000,000

For a 32,768 Hz crystal with +20 ppm error:

32,768 × 20 / 1,000,000

= 0.65536 Hz

The corresponding oscillator frequency is approximately 32,768.65536 Hz.

Why 32.768 kHz Is Used for RTCs

32.768 kHz is common in real-time clocks because 32,768 equals 2 to the power of 15. A binary divider can therefore reduce this frequency to a convenient one-second timing reference.

32,768 Hz
÷ 2¹⁵
= 1 Hz

Calculate PPM from Measured Clock Drift

If you measure an RTC against an accurate reference, the frequency error can be estimated from the observed time drift.

PPM =

Observed Drift
--------------
Elapsed Time

× 1,000,000

Both observed drift and elapsed time must use the same time units before applying the ratio.

Measured Drift Example

RTC gains:
12 seconds

Measurement period:
7 days

Elapsed seconds:
604,800

PPM:

12 / 604,800 × 1,000,000

≈ 19.8413 ppm

A compensating calibration would ideally apply approximately the opposite error, subject to the resolution and implementation of the RTC calibration hardware.

RTC Calibration Correction

If an RTC runs fast by +10 ppm, an ideal first-order frequency correction is approximately −10 ppm. If it runs slow by −10 ppm, the corresponding first-order correction is approximately +10 ppm.

Measured Error:
+12.5 ppm

Ideal Compensation:
-12.5 ppm

Actual calibration registers may have discrete step sizes and may add or remove periodic clock pulses rather than directly changing oscillator frequency.

Time to Accumulate One Second of Drift

The time required to accumulate a specified error can also be calculated from the absolute ppm error.

Elapsed Seconds =

Desired Drift × 1,000,000
-------------------------
|PPM|

At 20 ppm, approximately 50,000 seconds of real time are required to accumulate one second of clock error, or about 13.89 hours.

Crystal Temperature and RTC Accuracy

RTC drift is not necessarily constant over every operating condition. Quartz crystal frequency can change with temperature, load capacitance, aging, manufacturing tolerance and circuit layout.

A single ppm measurement therefore describes the average error during the measurement interval rather than guaranteeing identical drift under every future condition.

Crystal Aging

Crystal oscillators can experience gradual frequency change with age. This means a device calibrated during manufacturing can develop a different average clock error after months or years of operation.

Systems requiring tighter long-term accuracy may periodically synchronize to an external time reference or use a temperature-compensated RTC.

RTC Drift vs Temperature-Compensated RTC

A basic crystal-based RTC uses the natural oscillator frequency with limited or no active temperature correction. Temperature-compensated real-time clocks can measure temperature and apply internal frequency compensation.

Their achievable accuracy still depends on the specific component, temperature range and datasheet specification.

RTC Clock Drift Calculator FAQs

How much clock drift does 1 ppm cause per day?

One ppm corresponds to approximately 0.0864 seconds, or 86.4 milliseconds, of drift per day.

How much does a 20 ppm RTC drift per day?

The magnitude is approximately 1.728 seconds per day. Positive 20 ppm gains that amount; negative 20 ppm loses that amount.

How much does 20 ppm drift per year?

Over 365 days, 20 ppm corresponds to approximately 630.72 seconds, or about 10 minutes 30.72 seconds.

What does positive RTC ppm mean?

Positive ppm means the oscillator is faster than nominal and the RTC gains time.

What does negative ppm mean?

Negative ppm means the oscillator is slower than nominal and the RTC loses time.

How do I calculate ppm from measured drift?

Divide observed time drift by the elapsed reference time and multiply by one million.

Why are RTC crystals often 32.768 kHz?

32,768 is 2¹⁵, so a binary divider can efficiently derive a 1 Hz timing reference.

Can RTC drift change with temperature?

Yes. Quartz oscillator frequency is temperature-dependent, so average clock error can change as operating temperature changes.

Does the calculator include crystal aging?

No. The entered ppm is treated as a constant average frequency error for the calculation interval.

What calibration should I use for a +10 ppm RTC?

The ideal first-order correction is approximately −10 ppm, but the actual register setting depends on the RTC hardware’s calibration step size and method.

Calculate RTC Drift and Clock Accuracy

Convert oscillator ppm error into daily, monthly, annual and custom-interval time drift, frequency offset and ideal calibration correction for RTC and 32.768 kHz clock systems.