10進数の文字数と、16進数の文字数の比較表です。
（末尾に 'L' や 'LL' が付く場合は考慮していません）
10^13 から16進数の方が短めになるようです。
| 10進数 | 16進数 | |||
|---|---|---|---|---|
| 0 | 1 | < | 3 | 0x0 |
| 9 | 1 | < | 3 | 0x9 |
| 10 | 2 | < | 3 | 0xa |
| 15 | 2 | < | 3 | 0xf |
| 16 | 2 | < | 4 | 0x10 |
| 99 | 2 | < | 4 | 0x63 |
| 100 | 3 | < | 4 | 0x64 |
| 255 | 3 | < | 4 | 0xff |
| 256 | 3 | < | 5 | 0x100 |
| 999 | 3 | < | 5 | 0x3e7 |
| 1000 | 4 | < | 5 | 0x3e8 |
| 4095 | 4 | < | 5 | 0xfff |
| 4096 | 4 | < | 6 | 0x1000 |
| 9999 | 4 | < | 6 | 0x270f |
| 10000 | 5 | < | 6 | 0x2710 |
| 65535 | 5 | < | 6 | 0xffff |
| 65536 | 5 | < | 7 | 0x10000 |
| 99999 | 5 | < | 7 | 0x1869f |
| 100000 | 6 | < | 7 | 0x186a0 |
| 999999 | 6 | < | 7 | 0xf423f |
| 1000000 | 7 | = | 7 | 0xf4240 |
| 1048575 | 7 | = | 7 | 0xfffff |
| 1048576 | 7 | < | 8 | 0x100000 |
| 9999999 | 7 | < | 8 | 0x98967f |
| 10000000 | 8 | = | 8 | 0x989680 |
| 16777215 | 8 | = | 8 | 0xffffff |
| 16777216 | 8 | < | 9 | 0x1000000 |
| 99999999 | 8 | < | 9 | 0x5f5e0ff |
| 100000000 | 9 | = | 9 | 0x5f5e100 |
| 268435455 | 9 | = | 9 | 0xfffffff |
| 268435456 | 9 | < | 10 | 0x10000000 |
| 999999999 | 9 | < | 10 | 0x3b9ac9ff |
| 1000000000 | 10 | = | 10 | 0x3b9aca00 |
| 4294967295 | 10 | = | 10 | 0xffffffff |
| 4294967296 | 10 | < | 11 | 0x100000000 |
| 9999999999 | 10 | < | 11 | 0x2540be3ff |
| 10000000000 | 11 | = | 11 | 0x2540be400 |
| 68719476735 | 11 | = | 11 | 0xfffffffff |
| 68719476736 | 11 | < | 12 | 0x1000000000 |
| 99999999999 | 11 | < | 12 | 0x174876e7ff |
| 100000000000 | 12 | = | 12 | 0x174876e800 |
| 999999999999 | 12 | = | 12 | 0xe8d4a50fff |
| 1000000000000 | 13 | > | 12 | 0xe8d4a51000 |
| 1099511627775 | 13 | > | 12 | 0xffffffffff |
| 1099511627776 | 13 | = | 13 | 0x10000000000 |
| 9999999999999 | 13 | = | 13 | 0x9184e729fff |
| 10000000000000 | 14 | > | 13 | 0x9184e72a000 |
| 17592186044415 | 14 | > | 13 | 0xfffffffffff |
| 17592186044416 | 14 | = | 14 | 0x100000000000 |
| 99999999999999 | 14 | = | 14 | 0x5af3107a3fff |
| 100000000000000 | 15 | > | 14 | 0x5af3107a4000 |
| 281474976710655 | 15 | > | 14 | 0xffffffffffff |
| 281474976710656 | 15 | = | 15 | 0x1000000000000 |
| 999999999999999 | 15 | = | 15 | 0x38d7ea4c67fff |
| 1000000000000000 | 16 | > | 15 | 0x38d7ea4c68000 |
| 4503599627370495 | 16 | > | 15 | 0xfffffffffffff |
| 4503599627370496 | 16 | = | 16 | 0x10000000000000 |
| 9999999999999999 | 16 | = | 16 | 0x2386f26fc0ffff |
| 10000000000000000 | 17 | > | 16 | 0x2386f26fc10000 |
| 72057594037927935 | 17 | > | 16 | 0xffffffffffffff |
| 72057594037927936 | 17 | = | 17 | 0x100000000000000 |
| 99999999999999999 | 17 | = | 17 | 0x16345785d89ffff |
| 100000000000000000 | 18 | > | 17 | 0x16345785d8a0000 |
| 999999999999999999 | 18 | > | 17 | 0xde0b6b3a763ffff |
| 1000000000000000000 | 19 | > | 17 | 0xde0b6b3a7640000 |
| 1152921504606846975 | 19 | > | 17 | 0xfffffffffffffff |
| 1152921504606846976 | 19 | > | 18 | 0x1000000000000000 |
| 9999999999999999999 | 19 | > | 18 | 0x8ac7230489e7ffff |
| 10000000000000000000 | 20 | > | 18 | 0x8ac7230489e80000 |
| 18446744073709551615 | 20 | > | 18 | 0xffffffffffffffff |