diff --git a/src/console_gui.cpp b/src/console_gui.cpp
--- a/src/console_gui.cpp
+++ b/src/console_gui.cpp
@@ -133,7 +133,7 @@ IConsoleModes _iconsole_mode;
 static void IConsoleClearCommand()
 {
 	memset(_iconsole_cmdline.buf, 0, ICON_CMDLN_SIZE);
-	_iconsole_cmdline.length = 0;
+	_iconsole_cmdline.size = 1; // only terminating zero
 	_iconsole_cmdline.width = 0;
 	_iconsole_cmdline.caretpos = 0;
 	_iconsole_cmdline.caretxoffs = 0;
@@ -336,7 +336,7 @@ void IConsoleGUIInit()
 	memset(_iconsole_history, 0, sizeof(_iconsole_history));
 
 	_iconsole_cmdline.buf = CallocT<char>(ICON_CMDLN_SIZE); // create buffer and zero it
-	_iconsole_cmdline.maxlength = ICON_CMDLN_SIZE;
+	_iconsole_cmdline.maxsize = ICON_CMDLN_SIZE;
 
 	IConsolePrintF(CC_WARNING, "OpenTTD Game Console Revision 7 - %s", _openttd_revision);
 	IConsolePrint(CC_WHITE,  "------------------------------------");
@@ -430,7 +430,7 @@ static void IConsoleHistoryNavigate(int 
 	IConsoleClearCommand();
 	/* copy history to 'command prompt / bash' */
 	assert(_iconsole_history[i] != NULL && IsInsideMM(i, 0, ICON_HISTORY_SIZE));
-	ttd_strlcpy(_iconsole_cmdline.buf, _iconsole_history[i], _iconsole_cmdline.maxlength);
+	ttd_strlcpy(_iconsole_cmdline.buf, _iconsole_history[i], _iconsole_cmdline.maxsize);
 	UpdateTextBufferSize(&_iconsole_cmdline);
 }
 
diff --git a/src/genworld_gui.cpp b/src/genworld_gui.cpp
--- a/src/genworld_gui.cpp
+++ b/src/genworld_gui.cpp
@@ -255,6 +255,7 @@ struct GenerateLandscapeWindow : public 
 	{
 		this->LowerWidget(_settings_newgame.game_creation.landscape + GLAND_TEMPERATE);
 
+		/* snprintf() always outputs trailing '\0', so whole buffer can be used */
 		snprintf(this->edit_str_buf, this->edit_str_size, "%u", _settings_newgame.game_creation.generation_seed);
 		InitializeTextBuffer(&this->text, this->edit_str_buf, this->edit_str_size, 120);
 		this->caption = STR_NULL;
diff --git a/src/misc_gui.cpp b/src/misc_gui.cpp
--- a/src/misc_gui.cpp
+++ b/src/misc_gui.cpp
@@ -793,8 +793,8 @@ static void DelChar(Textbuf *tb, bool ba
 	}
 
 	/* Move the remaining characters over the marker */
-	memmove(s, s + len, tb->length - (s - tb->buf) - len + 1);
-	tb->length -= len;
+	memmove(s, s + len, tb->size - (s - tb->buf) - len);
+	tb->size -= len;
 }
 
 /**
@@ -809,7 +809,7 @@ bool DeleteTextBufferChar(Textbuf *tb, i
 	if (delmode == WKC_BACKSPACE && tb->caretpos != 0) {
 		DelChar(tb, true);
 		return true;
-	} else if (delmode == WKC_DELETE && tb->caretpos < tb->length) {
+	} else if (delmode == WKC_DELETE && tb->caretpos < tb->size - 1) {
 		DelChar(tb, false);
 		return true;
 	}
@@ -823,9 +823,9 @@ bool DeleteTextBufferChar(Textbuf *tb, i
  */
 void DeleteTextBufferAll(Textbuf *tb)
 {
-	memset(tb->buf, 0, tb->maxlength);
-	tb->length = tb->width = 0;
-	tb->caretpos = tb->caretxoffs = 0;
+	memset(tb->buf, 0, tb->maxsize);
+	tb->size = 1;
+	tb->width = tb->caretpos = tb->caretxoffs = 0;
 }
 
 /**
@@ -840,11 +840,11 @@ bool InsertTextBufferChar(Textbuf *tb, W
 {
 	const byte charwidth = GetCharacterWidth(FS_NORMAL, key);
 	uint16 len = (uint16)Utf8CharLen(key);
-	if (tb->length < (tb->maxlength - len) && (tb->maxwidth == 0 || tb->width + charwidth <= tb->maxwidth)) {
-		memmove(tb->buf + tb->caretpos + len, tb->buf + tb->caretpos, tb->length - tb->caretpos + 1);
+	if (tb->size + len <= tb->maxsize && (tb->maxwidth == 0 || tb->width + charwidth <= tb->maxwidth)) {
+		memmove(tb->buf + tb->caretpos + len, tb->buf + tb->caretpos, tb->size - tb->caretpos);
 		Utf8Encode(tb->buf + tb->caretpos, key);
-		tb->length += len;
-		tb->width  += charwidth;
+		tb->size  += len;
+		tb->width += charwidth;
 
 		tb->caretpos   += len;
 		tb->caretxoffs += charwidth;
@@ -876,7 +876,7 @@ bool MoveTextBufferPos(Textbuf *tb, int 
 			break;
 
 		case WKC_RIGHT:
-			if (tb->caretpos < tb->length) {
+			if (tb->caretpos < tb->size - 1) {
 				WChar c;
 
 				tb->caretpos   += (uint16)Utf8Decode(&c, tb->buf + tb->caretpos);
@@ -892,7 +892,7 @@ bool MoveTextBufferPos(Textbuf *tb, int 
 			return true;
 
 		case WKC_END:
-			tb->caretpos = tb->length;
+			tb->caretpos = tb->size - 1;
 			tb->caretxoffs = tb->width;
 			return true;
 
@@ -908,16 +908,18 @@ bool MoveTextBufferPos(Textbuf *tb, int 
  * and the maximum length of this buffer
  * @param tb Textbuf type which is getting initialized
  * @param buf the buffer that will be holding the data for input
- * @param maxlength maximum length in characters of this buffer
+ * @param maxsize maximum size in bytes, including terminating '\0'
  * @param maxwidth maximum length in pixels of this buffer. If reached, buffer
- * cannot grow, even if maxlength would allow because there is space. A length
- * of zero '0' means the buffer is only restricted by maxlength */
-void InitializeTextBuffer(Textbuf *tb, const char *buf, uint16 maxlength, uint16 maxwidth)
+ * cannot grow, even if maxsize would allow because there is space. Width
+ * of zero '0' means the buffer is only restricted by maxsize */
+void InitializeTextBuffer(Textbuf *tb, char *buf, uint16 maxsize, uint16 maxwidth)
 {
-	tb->buf = (char*)buf;
-	tb->maxlength = maxlength;
-	tb->maxwidth  = maxwidth;
-	tb->caret = true;
+	assert(maxsize != 0);
+
+	tb->buf      = buf;
+	tb->maxsize  = maxsize;
+	tb->maxwidth = maxwidth;
+	tb->caret    = true;
 	UpdateTextBufferSize(tb);
 }
 
@@ -930,17 +932,19 @@ void InitializeTextBuffer(Textbuf *tb, c
 void UpdateTextBufferSize(Textbuf *tb)
 {
 	const char *buf = tb->buf;
-	WChar c = Utf8Consume(&buf);
 
 	tb->width = 0;
-	tb->length = 0;
+	tb->size = 1; // terminating zero
 
-	for (; c != '\0' && tb->length < (tb->maxlength - 1); c = Utf8Consume(&buf)) {
+	WChar c;
+	while ((c = Utf8Consume(&buf)) != '\0') {
 		tb->width += GetCharacterWidth(FS_NORMAL, c);
-		tb->length += Utf8CharLen(c);
+		tb->size += Utf8CharLen(c);
 	}
 
-	tb->caretpos = tb->length;
+	assert(tb->size <= tb->maxsize);
+
+	tb->caretpos = tb->size - 1;
 	tb->caretxoffs = tb->width;
 }
 
@@ -1163,22 +1167,22 @@ static const WindowDesc _query_string_de
 /** Show a query popup window with a textbox in it.
  * @param str StringID for the text shown in the textbox
  * @param caption StringID of text shown in caption of querywindow
- * @param maxlen maximum length in characters allowed
+ * @param maxsize maximum size in bytes (including terminating '\0')
  * @param maxwidth maximum width in pixels allowed
  * @param parent pointer to a Window that will handle the events (ok/cancel) of this
  *        window. If NULL, results are handled by global function HandleOnEditText
  * @param afilter filters out unwanted character input
  * @param flags various flags, @see QueryStringFlags
  */
-void ShowQueryString(StringID str, StringID caption, uint maxlen, uint maxwidth, Window *parent, CharSetFilter afilter, QueryStringFlags flags)
+void ShowQueryString(StringID str, StringID caption, uint maxsize, uint maxwidth, Window *parent, CharSetFilter afilter, QueryStringFlags flags)
 {
 	DeleteWindowById(WC_QUERY_STRING, 0);
 	DeleteWindowById(WC_SAVELOAD, 0);
 
-	QueryStringWindow *w = new QueryStringWindow(maxlen + 1, &_query_string_desc, parent);
+	QueryStringWindow *w = new QueryStringWindow(maxsize, &_query_string_desc, parent);
 
-	GetString(w->edit_str_buf, str, &w->edit_str_buf[maxlen]);
-	w->edit_str_buf[maxlen] = '\0';
+	GetString(w->edit_str_buf, str, &w->edit_str_buf[maxsize - 1]);
+	w->edit_str_buf[maxsize - 1] = '\0';
 
 	if ((flags & QSF_ACCEPT_UNCHANGED) == 0) w->orig = strdup(w->edit_str_buf);
 
@@ -1194,7 +1198,7 @@ void ShowQueryString(StringID str, Strin
 	w->LowerWidget(QUERY_STR_WIDGET_TEXT);
 	w->caption = caption;
 	w->afilter = afilter;
-	InitializeTextBuffer(&w->text, w->edit_str_buf, maxlen, maxwidth);
+	InitializeTextBuffer(&w->text, w->edit_str_buf, maxsize, maxwidth);
 }
 
 
@@ -1583,7 +1587,7 @@ struct SaveLoadWindow : public QueryStri
 						ShowHeightmapLoad();
 					} else {
 						/* SLD_SAVE_GAME, SLD_SAVE_SCENARIO copy clicked name to editbox */
-						ttd_strlcpy(this->text.buf, file->title, this->text.maxlength);
+						ttd_strlcpy(this->text.buf, file->title, this->text.maxsize);
 						UpdateTextBufferSize(&this->text);
 						this->InvalidateWidget(10);
 					}
diff --git a/src/network/network_chat_gui.cpp b/src/network/network_chat_gui.cpp
--- a/src/network/network_chat_gui.cpp
+++ b/src/network/network_chat_gui.cpp
@@ -374,11 +374,11 @@ struct NetworkChatWindow : public QueryS
 					/* If we are completing at the begin of the line, skip the ': ' we added */
 					if (tb_buf == pre_buf) {
 						offset = 0;
-						length = tb->length - 2;
+						length = (tb->size - 1) - 2;
 					} else {
 						/* Else, find the place we are completing at */
 						offset = strlen(pre_buf) + 1;
-						length = tb->length - offset;
+						length = (tb->size - 1) - offset;
 					}
 
 					/* Compare if we have a match */
diff --git a/src/querystring_gui.h b/src/querystring_gui.h
--- a/src/querystring_gui.h
+++ b/src/querystring_gui.h
@@ -41,10 +41,11 @@ struct QueryString {
 struct QueryStringBaseWindow : public Window, public QueryString {
 	char *edit_str_buf;
 	char *orig_str_buf;
-	const uint16 edit_str_size;
+	const uint16 edit_str_size; ///< maximum length of string (in bytes), including terminating '\0'
 
 	QueryStringBaseWindow(uint16 size, const WindowDesc *desc, WindowNumber window_number = 0) : Window(desc, window_number), edit_str_size(size)
 	{
+		assert(size != 0);
 		this->edit_str_buf = CallocT<char>(size);
 	}
 
diff --git a/src/signs_gui.cpp b/src/signs_gui.cpp
--- a/src/signs_gui.cpp
+++ b/src/signs_gui.cpp
@@ -215,7 +215,7 @@ struct SignWindow : QueryStringBaseWindo
 
 	void UpdateSignEditWindow(const Sign *si)
 	{
-		char *last_of = &this->edit_str_buf[this->edit_str_size - 1];
+		char *last_of = &this->edit_str_buf[this->edit_str_size - 1]; // points to terminating '\0'
 
 		/* Display an empty string when the sign hasnt been edited yet */
 		if (si->name != NULL) {
diff --git a/src/textbuf_gui.h b/src/textbuf_gui.h
--- a/src/textbuf_gui.h
+++ b/src/textbuf_gui.h
@@ -11,12 +11,12 @@
 #include "core/enum_type.hpp"
 
 struct Textbuf {
-	char *buf;                  ///< buffer in which text is saved
-	uint16 maxlength, maxwidth; ///< the maximum size of the buffer. Maxwidth specifies screensize in pixels, maxlength is in bytes
-	uint16 length, width;       ///< the current size of the string. Width specifies screensize in pixels, length is in bytes
-	bool caret;                 ///< is the caret ("_") visible or not
-	uint16 caretpos;            ///< the current position of the caret in the buffer, in bytes
-	uint16 caretxoffs;          ///< the current position of the caret in pixels
+	char *buf;                ///< buffer in which text is saved
+	uint16 maxsize, maxwidth; ///< the maximum size of the buffer. Maxwidth specifies screensize in pixels, maxsize is in bytes (including terminating '\0')
+	uint16 size, width;       ///< the current size of the string. Width specifies screensize in pixels, size is in bytes
+	bool caret;               ///< is the caret ("_") visible or not
+	uint16 caretpos;          ///< the current position of the caret in the buffer, in bytes
+	uint16 caretxoffs;        ///< the current position of the caret in pixels
 };
 
 bool HandleCaret(Textbuf *tb);
@@ -26,7 +26,7 @@ bool DeleteTextBufferChar(Textbuf *tb, i
 bool InsertTextBufferChar(Textbuf *tb, uint32 key);
 bool InsertTextBufferClipboard(Textbuf *tb);
 bool MoveTextBufferPos(Textbuf *tb, int navmode);
-void InitializeTextBuffer(Textbuf *tb, const char *buf, uint16 maxlength, uint16 maxwidth);
+void InitializeTextBuffer(Textbuf *tb, char *buf, uint16 maxsize, uint16 maxwidth);
 void UpdateTextBufferSize(Textbuf *tb);
 
 /** Flags used in ShowQueryString() call */