diff --git a/esphome/components/esp8266/__init__.py b/esphome/components/esp8266/__init__.py
index d95cbd239b5..6088c20b520 100644
--- a/esphome/components/esp8266/__init__.py
+++ b/esphome/components/esp8266/__init__.py
@@ -53,6 +53,10 @@ from .const import (
     KEY_SERIAL1_REQUIRED,
     KEY_SERIAL_REQUIRED,
     KEY_WAVEFORM_REQUIRED,
+    PREFERENCES_FLASH_WORDS,
+    PREFERENCES_FLASH_WORDS_WITHOUT_RESTORE,
+    PREFERENCES_RTC_EBOOT_WORDS,
+    PREFERENCES_RTC_NORMAL_WORDS,
     THROW_STUBS_HEADER,
     enable_serial,
     enable_serial1,
@@ -467,8 +471,16 @@ async def to_code(config: ConfigType) -> None:
     # Other projects like Tasmota & ESPEasy also use this
     cg.add_build_flag("-DPIO_FRAMEWORK_ARDUINO_LWIP2_HIGHER_BANDWIDTH_LOW_FLASH")
 
+    # Saved-settings layout, written down once in const.py and read by preferences.cpp
+    cg.add_define("ESP8266_PREFERENCES_RTC_NORMAL_WORDS", PREFERENCES_RTC_NORMAL_WORDS)
+    cg.add_define("ESP8266_PREFERENCES_RTC_EBOOT_WORDS", PREFERENCES_RTC_EBOOT_WORDS)
     if config[CONF_RESTORE_FROM_FLASH]:
         cg.add_define("USE_ESP8266_PREFERENCES_FLASH")
+        cg.add_define("ESP8266_PREFERENCES_FLASH_WORDS", PREFERENCES_FLASH_WORDS)
+    else:
+        cg.add_define(
+            "ESP8266_PREFERENCES_FLASH_WORDS", PREFERENCES_FLASH_WORDS_WITHOUT_RESTORE
+        )
 
     if config[CONF_EARLY_PIN_INIT]:
         cg.add_define("USE_ESP8266_EARLY_PIN_INIT")
diff --git a/esphome/components/esp8266/const.py b/esphome/components/esp8266/const.py
index a702b397049..7926bf84b28 100644
--- a/esphome/components/esp8266/const.py
+++ b/esphome/components/esp8266/const.py
@@ -24,11 +24,35 @@ KEY_LDSCRIPT = "ldscript"
 # Crash-decode tools, resolved once per run by __init__
 KEY_DECODE_TOOLS = "decode_tools"
 
-# Space for saved settings, in 4-byte words, from preferences.cpp: RTC memory has two regions
-# (RTC_NORMAL_REGION_WORDS, RTC_EBOOT_REGION_WORDS) that one saved value cannot span; with
-# restore_from_flash everything goes to flash (ESP8266_FLASH_STORAGE_SIZE)
-PREFERENCES_RTC_REGION_WORDS = (78, 32)
+# Space for saved settings, in 4-byte words. Codegen hands these to preferences.cpp as defines, so this is the
+# only place they are written down. RTC memory has two regions that one saved value cannot span, the normal one
+# and the one eboot also uses; crash_handler.cpp keeps the last 18 of the 128 RTC words. The flash store holds
+# every saved value with restore_from_flash and only the explicit ones without, so it is smaller then.
+PREFERENCES_RTC_NORMAL_WORDS = 78
+PREFERENCES_RTC_EBOOT_WORDS = 32
 PREFERENCES_FLASH_WORDS = 128
+PREFERENCES_FLASH_WORDS_WITHOUT_RESTORE = 64
+
+
+def preference_words(value_bytes: int) -> int:
+    """Words one saved value of this size takes, including its checksum word."""
+    return (value_bytes + 3) // 4 + 1
+
+
+def preference_slots(value_bytes: int, restore_from_flash: bool) -> int:
+    """How many saved values of this size fit when every slot holds one of them.
+
+    With restore_from_flash they all go to the flash store; otherwise they go to RTC memory, where a value
+    cannot straddle the two regions.
+    """
+    words = preference_words(value_bytes)
+    regions = (
+        (PREFERENCES_FLASH_WORDS,)
+        if restore_from_flash
+        else (PREFERENCES_RTC_NORMAL_WORDS, PREFERENCES_RTC_EBOOT_WORDS)
+    )
+    return sum(region // words for region in regions)
+
 
 # esp8266 namespace is already defined by arduino, manually prefix esphome
 esp8266_ns = cg.global_ns.namespace("esphome").namespace("esp8266")
diff --git a/esphome/components/esp8266/preferences.cpp b/esphome/components/esp8266/preferences.cpp
index dbfb233a1d4..39ef000e350 100644
--- a/esphome/components/esp8266/preferences.cpp
+++ b/esphome/components/esp8266/preferences.cpp
@@ -20,29 +20,23 @@ static constexpr uint32_t ESP_RTC_USER_MEM_START = 0x60001200;
 static constexpr uint32_t ESP_RTC_USER_MEM_SIZE_WORDS = 128;
 static constexpr uint32_t ESP_RTC_USER_MEM_SIZE_BYTES = ESP_RTC_USER_MEM_SIZE_WORDS * 4;
 
-// RTC memory layout:
+// RTC memory layout, with the sizes defined once in const.py and handed over by codegen:
 // - Eboot region:  RTC words 0-31   (reserved, mapped from preference offset 78-109)
 // - Normal region: RTC words 32-109 (mapped from preference offset 0-77)
 // - Crash handler: RTC words 110-127 (reserved for crash_handler.cpp backtrace data)
-// const.py mirrors the region and flash sizes for config-time space checks
-static constexpr uint32_t RTC_EBOOT_REGION_WORDS = 32;   // Words 0-31 reserved for eboot
-static constexpr uint32_t RTC_NORMAL_REGION_WORDS = 78;  // Words 32-109 for normal prefs
+static constexpr uint32_t RTC_EBOOT_REGION_WORDS = ESP8266_PREFERENCES_RTC_EBOOT_WORDS;
+static constexpr uint32_t RTC_NORMAL_REGION_WORDS = ESP8266_PREFERENCES_RTC_NORMAL_WORDS;
 static constexpr uint32_t PREF_TOTAL_WORDS = RTC_EBOOT_REGION_WORDS + RTC_NORMAL_REGION_WORDS;  // 110
+static_assert(PREF_TOTAL_WORDS <= ESP_RTC_USER_MEM_SIZE_WORDS, "preference regions overflow RTC memory");
 
 // Maximum preference size in words (limited by uint8_t length_words field)
 static constexpr uint32_t MAX_PREFERENCE_WORDS = 255;
 
 #define ESP_RTC_USER_MEM ((uint32_t *) ESP_RTC_USER_MEM_START)
 
-// Flash storage size depends on esp8266 -> restore_from_flash YAML option (default: false).
-// When enabled (USE_ESP8266_PREFERENCES_FLASH), all preferences default to flash and need
-// 128 words (512 bytes). When disabled, only explicit flash prefs use this storage so
-// 64 words (256 bytes) suffices since most preferences go to RTC memory instead.
-#ifdef USE_ESP8266_PREFERENCES_FLASH
-static constexpr uint32_t ESP8266_FLASH_STORAGE_SIZE = 128;
-#else
-static constexpr uint32_t ESP8266_FLASH_STORAGE_SIZE = 64;
-#endif
+// With restore_from_flash every preference goes here; without it only the explicit flash ones do,
+// so codegen hands over a smaller size (const.py).
+static constexpr uint32_t ESP8266_FLASH_STORAGE_SIZE = ESP8266_PREFERENCES_FLASH_WORDS;
 
 static uint32_t
     s_flash_storage[ESP8266_FLASH_STORAGE_SIZE];  // NOLINT(cppcoreguidelines-avoid-non-const-global-variables)
@@ -124,7 +118,7 @@ static bool load_from_rtc(size_t offset, uint32_t *data, size_t len) {
 
 // Maximum buffer for any single preference - bounded by storage sizes.
 // Flash prefs: bounded by ESP8266_FLASH_STORAGE_SIZE (128 or 64 words).
-// RTC prefs: bounded by RTC_NORMAL_REGION_WORDS (96) - a single pref can't span both RTC regions.
+// RTC prefs: bounded by RTC_NORMAL_REGION_WORDS (78) - a single pref can't span both RTC regions.
 static constexpr size_t PREF_MAX_BUFFER_WORDS =
     ESP8266_FLASH_STORAGE_SIZE > RTC_NORMAL_REGION_WORDS ? ESP8266_FLASH_STORAGE_SIZE : RTC_NORMAL_REGION_WORDS;
 
@@ -181,8 +175,8 @@ ESPPreferenceObject ESP8266Preferences::make_preference(size_t length, uint32_t
   } else {
     uint32_t start = this->current_offset;
     bool in_normal = start < RTC_NORMAL_REGION_WORDS;
-    // Normal: offset 0-95 maps to RTC offset 32-127
-    // Eboot: offset 96-127 maps to RTC offset 0-31
+    // Normal: offset 0-77 maps to RTC offset 32-109
+    // Eboot: offset 78-109 maps to RTC offset 0-31
     if (in_normal && start + total_words > RTC_NORMAL_REGION_WORDS) {
       // start is in normal but end is not -> switch to Eboot
       this->current_offset = start = RTC_NORMAL_REGION_WORDS;
diff --git a/esphome/core/defines.h b/esphome/core/defines.h
index 8d243043bde..7a10abe3a3c 100644
--- a/esphome/core/defines.h
+++ b/esphome/core/defines.h
@@ -597,6 +597,9 @@
 #define USE_ESP8266_LOGGER_SERIAL
 #define USE_ESP8266_LOGGER_SERIAL1
 #define USE_ESP8266_PREFERENCES_FLASH
+#define ESP8266_PREFERENCES_FLASH_WORDS 128
+#define ESP8266_PREFERENCES_RTC_EBOOT_WORDS 32
+#define ESP8266_PREFERENCES_RTC_NORMAL_WORDS 78
 #define USE_ESP8266_UART_SERIAL
 #define USE_ESP8266_UART_SERIAL1
 #define USE_HTTP_REQUEST_ESP8266_HTTPS