Calculator
CRL to Gestational Age
Convert crown-rump length (CRL) to gestational age using the Robinson & Fleming (1975) formula — the international clinical standard for first-trimester dating.
Crown-Rump Length (CRL)
GA Dating—
Gestational Age
CRL Growth Chart
Reference Table
CRL to Gestational Age Reference
Complete reference table mapping crown-rump length (CRL) to gestational age using the Robinson & Fleming (1975) formula.
| CRL (mm) | Gestational Age | Pregnancy Week | Pregnancy Month |
|---|---|---|---|
| 1 | 4w4d | 4 | 1st |
| 2 | 5w0d | 5 | 2nd |
| 3 | 5w3d | 5 | 2nd |
| 4 | 5w5d | 5 | 2nd |
| 5 | 6w0d | 6 | 2nd |
| 6 | 6w1d | 6 | 2nd |
| 7 | 6w3d | 6 | 2nd |
| 8 | 6w5d | 6 | 2nd |
| 9 | 6w6d | 6 | 2nd |
| 10 | 7w0d | 7 | 2nd |
| 11 | 7w1d | 7 | 2nd |
| 12 | 7w3d | 7 | 2nd |
| 13 | 7w4d | 7 | 2nd |
| 14 | 7w5d | 7 | 2nd |
| 15 | 7w6d | 7 | 2nd |
| 16 | 8w0d | 8 | 2nd |
| 17 | 8w1d | 8 | 2nd |
| 18 | 8w2d | 8 | 2nd |
| 19 | 8w3d | 8 | 2nd |
| 20 | 8w4d | 8 | 2nd |
| 21 | 8w5d | 8 | 2nd |
| 22 | 8w5d | 8 | 2nd |
| 23 | 8w6d | 8 | 2nd |
| 24 | 9w0d | 9 | 3rd |
| 25 | 9w1d | 9 | 3rd |
| 26 | 9w2d | 9 | 3rd |
| 27 | 9w3d | 9 | 3rd |
| 28 | 9w3d | 9 | 3rd |
| 29 | 9w4d | 9 | 3rd |
| 30 | 9w5d | 9 | 3rd |
| 31 | 9w6d | 9 | 3rd |
| 32 | 9w6d | 9 | 3rd |
| 33 | 10w0d | 10 | 3rd |
| 34 | 10w1d | 10 | 3rd |
| 35 | 10w1d | 10 | 3rd |
| 36 | 10w2d | 10 | 3rd |
| 37 | 10w3d | 10 | 3rd |
| 38 | 10w3d | 10 | 3rd |
| 39 | 10w4d | 10 | 3rd |
| 40 | 10w5d | 10 | 3rd |
| 41 | 10w5d | 10 | 3rd |
| 42 | 10w6d | 10 | 3rd |
| 43 | 11w0d | 11 | 3rd |
| 44 | 11w0d | 11 | 3rd |
| 45 | 11w1d | 11 | 3rd |
| 46 | 11w1d | 11 | 3rd |
| 47 | 11w2d | 11 | 3rd |
| 48 | 11w3d | 11 | 3rd |
| 49 | 11w3d | 11 | 3rd |
| 50 | 11w4d | 11 | 3rd |
| 51 | 11w4d | 11 | 3rd |
| 52 | 11w5d | 11 | 3rd |
| 53 | 11w5d | 11 | 3rd |
| 54 | 11w6d | 11 | 3rd |
| 55 | 11w6d | 11 | 3rd |
| 56 | 12w0d | 12 | 3rd |
| 57 | 12w1d | 12 | 3rd |
| 58 | 12w1d | 12 | 3rd |
| 59 | 12w2d | 12 | 3rd |
| 60 | 12w2d | 12 | 3rd |
| 61 | 12w3d | 12 | 3rd |
| 62 | 12w3d | 12 | 3rd |
| 63 | 12w4d | 12 | 3rd |
| 64 | 12w4d | 12 | 3rd |
| 65 | 12w5d | 12 | 3rd |
| 66 | 12w5d | 12 | 3rd |
| 67 | 12w6d | 12 | 3rd |
| 68 | 12w6d | 12 | 3rd |
| 69 | 13w0d | 13 | 3rd |
| 70 | 13w0d | 13 | 3rd |
| 71 | 13w1d | 13 | 3rd |
| 72 | 13w1d | 13 | 3rd |
| 73 | 13w2d | 13 | 3rd |
| 74 | 13w2d | 13 | 3rd |
| 75 | 13w2d | 13 | 3rd |
| 76 | 13w3d | 13 | 3rd |
| 77 | 13w3d | 13 | 3rd |
| 78 | 13w4d | 13 | 3rd |
| 79 | 13w4d | 13 | 3rd |
| 80 | 13w5d | 13 | 3rd |
| 81 | 13w5d | 13 | 3rd |
| 82 | 13w6d | 13 | 3rd |
| 83 | 13w6d | 13 | 3rd |
| 84 | 14w0d | 14 | 4th |
| 85 | 14w0d | 14 | 4th |
| 86 | 14w0d | 14 | 4th |
| 87 | 14w1d | 14 | 4th |
| 88 | 14w1d | 14 | 4th |
| 89 | 14w2d | 14 | 4th |
| 90 | 14w2d | 14 | 4th |
| 91 | 14w3d | 14 | 4th |
| 92 | 14w3d | 14 | 4th |
| 93 | 14w3d | 14 | 4th |
| 94 | 14w4d | 14 | 4th |
| 95 | 14w4d | 14 | 4th |
| 96 | 14w5d | 14 | 4th |
| 97 | 14w5d | 14 | 4th |
| 98 | 14w5d | 14 | 4th |
| 99 | 14w6d | 14 | 4th |
| 100 | 14w6d | 14 | 4th |
| 101 | 15w0d | 15 | 4th |
| 102 | 15w0d | 15 | 4th |
| 103 | 15w0d | 15 | 4th |
| 104 | 15w1d | 15 | 4th |
| 105 | 15w1d | 15 | 4th |
| 106 | 15w2d | 15 | 4th |
| 107 | 15w2d | 15 | 4th |
| 108 | 15w2d | 15 | 4th |
| 109 | 15w3d | 15 | 4th |
| 110 | 15w3d | 15 | 4th |
| 111 | 15w4d | 15 | 4th |
| 112 | 15w4d | 15 | 4th |
| 113 | 15w4d | 15 | 4th |
| 114 | 15w5d | 15 | 4th |
| 115 | 15w5d | 15 | 4th |
| 116 | 15w5d | 15 | 4th |
| 117 | 15w6d | 15 | 4th |
| 118 | 15w6d | 15 | 4th |
| 119 | 16w0d | 16 | 4th |
| 120 | 16w0d | 16 | 4th |
Values computed using the Robinson & Fleming (1975) formula: GA (days) = 8.052 × √CRL + 23.73. Clinically validated range: 5–84 mm. Rows outside this range are shown for reference only.
About This Calculator
Method
The Robinson & Fleming formula was derived from serial ultrasound measurements in pregnancies with precisely known conception dates. A square-root transformation was applied to linearize the embryonic growth curve. The resulting regression achieved a high coefficient of determination, confirming CRL as a strong predictor of gestational age in the first trimester.
Valid Range
CRL measurements between 5 mm and 84 mm (approximately 6 weeks 2 days to 14 weeks 1 day). Peak accuracy occurs at 11–50 mm (7 weeks 0 days to 11 weeks 4 days), where the 95% confidence interval is approximately ±3 days.
Measurements below 5 mm are unreliable. Above 84 mm (approximately 14 weeks), switch to head circumference (HC) or biparietal diameter (BPD) for dating.
Accuracy
| CRL Range | GA Range | 95% CI |
|---|---|---|
| < 5 mm | Before ≈6w2d | Not reliable |
| 5–10 mm | 6w2d–6w6d | ±4 days |
| 11–50 mm | 7w0d–11w4d | ±3 days |
| 51–84 mm | 11w5d–14w1d | ±5 days |
| > 84 mm | Beyond ≈14w1d | Not applicable |
Limitations
- Requires a true mid-sagittal section with neutral fetal flexion. Off-angle images introduce error.
- Derived from predominantly European/North American cohorts; minor population variation may apply.
- Maternal obesity can degrade image quality and widen the confidence interval.
- Chromosomal anomalies (trisomy 18, triploidy) may reduce CRL.
- Maternal age and smoking affect growth beyond 10 weeks.
- IVF pregnancies should be dated from embryo transfer, not CRL.
- Transabdominal ultrasound may underestimate gestational age by ~1.6 days compared to transvaginal.
Sources
Robinson HP, Fleming JEE. A critical evaluation of sonar "crown-rump length" measurements. Br J Obstet Gynaecol. 1975;82(9):702-710. PMID: 1182090.
ACOG Committee Opinion No. 700. Methods for estimating the due date. Obstet Gynecol. 2017;129(5):e150-e154. PMID: 28426621.
Salomon LJ et al.; ISUOG. ISUOG practice guidelines: performance of first-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol. 2013;41(1):102-113. PMID: 23280739.
Verburg BO et al. New charts for ultrasound dating of pregnancy and assessment of fetal growth. Ultrasound Obstet Gynecol. 2008;31(4):388-396. PMID: 18348183.
Westerway SC, Davison A, Cowell S. Ultrasonic fetal measurements: new Australian standards for the new millennium. Aust N Z J Obstet Gynaecol. 2000;40(3):297-302.