Why can't I convert curies to rad with a single fixed number?

Curie measures radioactivity (decays per second); rad measures energy absorbed by tissue. The connection depends on how much energy each decay deposits, how much tissue absorbs it, and for how long — none of which are fixed by the activity alone. This tool makes all three factors explicit.

What is the exact Ci to Bq conversion?

1 curie (Ci) = 3.7 × 10^10 becquerels (Bq). This is an exact, defined relationship. The curie was originally set to the activity of 1 gram of radium-226, which happens to be 37 gigabecquerels.

What is 1 rad in SI units?

1 rad = 0.01 gray (Gy) = 0.01 joules of energy absorbed per kilogram of tissue. The gray is the modern SI unit; rad (radiation absorbed dose) remains common in US medical and industrial practice.

How does radiation type affect the result?

Different particles deposit different energies per decay. Alpha particles typically release ~5 MeV, beta particles average ~0.5 MeV, and gamma/X-rays vary by isotope. The tool uses representative mean values; for precision dosimetry use isotope-specific data.

What is the difference between rad and rem (or gray and sievert)?

Rad and gray measure absorbed dose — purely physical energy deposition. Rem and sievert measure effective dose, which multiplies absorbed dose by a radiation weighting factor (RBE) to reflect biological harm. Alpha radiation has an RBE of ~20, so 1 rad of alpha causes ~20× more biological damage than 1 rad of gamma.

What assumptions does this calculation make?

The tool assumes uniform irradiation, complete absorption of all emitted energy within the stated mass, and representative mean energies per decay. Real dosimetry requires geometry corrections, shielding data, and isotope-specific decay spectra for accurate results.

Curies to Rad Converter

Enter a curie value, pick a radiation type, tissue mass, and exposure time to see the full Ci → Bq → Gy → rad conversion chain.

Input Parameters

Curie Prefix

Activity

= 3.7 × 10¹⁰ Bq (37 GBq)

Radiation Type

Mean energy deposited per decay. Use isotope-specific data for precision.

Absorbing Mass

Typical whole-body mass: 70 kg = 70,000 g.

Exposure Duration

Absorbed Dose

Absorbed Dose (rad)

—

Absorbed Dose (gray)

—

Energy per decay used

—

Conversion Chain

Ci value: —

↓ × 3.7×10¹⁰

Bq: —

↓ × E/decay × time ÷ mass

Gy: —

↓ × 100

rad: —

Reference Doses

Chest X-ray	~0.001 rad
Annual background (USA)	~0.3 rad/yr
CT scan (abdomen)	~1 rad
Acute threshold (short exposure)	~25 rad

How the calculation works

Step 1: Convert to Bq — Activity (Bq) = Ci value × 3.7 × 10¹⁰. Step 2: Total decays = Activity (Bq) × Time (s). Step 3: Total energy deposited (J) = Total decays × Energy per decay (J). Step 4: Absorbed dose (Gy) = Total energy ÷ Mass (kg). Step 5: Absorbed dose (rad) = Gray × 100.

Assumes complete energy absorption within the stated mass with no geometry or shielding corrections. For precision dosimetry, use Monte Carlo transport codes or vendor-supplied dose-conversion factors.

Summary

Curies measure radioactivity (decays per second), while rad measures absorbed dose (energy deposited per kilogram of tissue, in CGS units). Converting curies to rad requires two bridging steps: first convert curies to becquerels (1 Ci = 3.7 × 10^10 Bq), then calculate absorbed dose using the radiation type, tissue mass, and exposure duration. This tool walks through all three steps and shows intermediate values so you can verify each stage.

How it works

Select the curie prefix (MCi, kCi, Ci, mCi, uCi, or nCi) and enter the activity value.
The tool converts the input to becquerels exactly: 1 Ci = 3.7 × 10^10 Bq.
Select the radiation type to apply the correct mean energy per decay (MeV).
Enter the absorbing tissue mass in grams or kilograms.
Set the exposure duration (seconds, minutes, hours, days, or years).
Absorbed dose in gray = (Activity × Energy/decay × Time) ÷ Mass; rad = gray × 100.

Use cases

Estimate tissue dose from a curie-rated radioactive source in a lab or clinical setting.
Verify manual dosimetry calculations in health physics or medical physics courses.
Understand the step-by-step unit chain from activity to absorbed dose.
Convert published nuclear medicine activities (given in mCi or uCi) to approximate dose figures.
Compare absorbed doses from different radiation types at the same curie level.
Cross-check radiation safety thresholds when working with legacy curie-rated sources.

Frequently Asked Questions

Last updated: 2026-07-14 · Reviewed by Nham Vu