Background: Accurate tip positioning of a peripherally inserted central catheter (PICC) is crucial for optimal drug delivery and avoiding complications. The objective of this study was to evaluate the amplitude ratios of intravascular electrocardiography (ivECG) and external electrocardiography (exECG) according to the tip location. Methods: This retrospective study analyzed ivECG, exECG, and chest X-ray (CXR) of 278 patients who underwent a PICC procedure. The tip-to-carina distance (TCD) was measured using vertebral body units (VBU) on CXR. Tip locations were categorized as follows: Zone 1, malposition (TCD < 0.8 VBU); Zone 2, suboptimal (0.8 VBU ≤ TCD < 1.5 VBU); Zone 3, optimal (1.5 VBU ≤ TCD ≤ 2.4 VBU); Zone 4, deep (TCD > 2.4 VBU). The amplitude ratios between ivECG and exECG and within ivECG were compared in each zone. Results: The ivECG/exECG amplitude ratios of P-wave (Piv/Pex) and QRS-complex (QRiv/QRex and RSiv/RSex) in Zone 3 were significantly higher than in Zones 1 and 2 (adjusted P < 0.05). The ivECG amplitude ratios of the P-wave and QRS-complex (Piv/QRiv and Piv/RSiv) were significantly lower in Zone 3 than in Zones 1 and 2 (adjusted P < 0.001). The calculated TCD using stepwise multiple regression analysis was estimated to be 1.121 + 0.078 × Piv/Pex – 0.172 × Piv/QRiv. Conclusions Though caution is required, amplitude ratios such as Piv/Pex and Piv/QRiv can help determine tip location during the PICC catheterization procedure.