diff --git a/davtelepot/messages.py b/davtelepot/messages.py
index 9b0921a..717fc5c 100644
--- a/davtelepot/messages.py
+++ b/davtelepot/messages.py
@@ -1024,6 +1024,10 @@ default_useful_tools_messages = {
                   "ottenerne il risultato, oppure usa il comando in risposta, "
                   "o ancora usa la tastiera qui sotto.\n",
         },
+        'invalid_expression': {
+            'en': "Invalid expression: {error}",
+            'it': "Espressione non valida: {error}",
+        },
         'language_labelled_commands': {
             'en': "calculate",
             'it': "calcola",
@@ -1036,6 +1040,14 @@ default_useful_tools_messages = {
                   "<i>L'espressione verrà mostrata quando smetterai di "
                   "digitare per un po'.</i>",
         },
+        'result': {
+            'en': "🔢 <b>Calculator</b> 🧮\n\n"
+                  "<i>Expressions evaluation:</i>\n\n"
+                  "{expressions}",
+            'it': "🔢 <b>Calcolatrice</b> 🧮\n\n"
+                  "<i>Risultato delle espresisoni:</i>\n\n"
+                  "{expressions}",
+        },
     },
     'info_command': {
         'description': {
diff --git a/davtelepot/useful_tools.py b/davtelepot/useful_tools.py
index bb9e600..dade6ee 100644
--- a/davtelepot/useful_tools.py
+++ b/davtelepot/useful_tools.py
@@ -1,10 +1,12 @@
 """General purpose functions for Telegram bots."""
 
 # Standard library
+import ast
 import asyncio
 import datetime
 import json
 import logging
+import operator
 
 from collections import OrderedDict
 from typing import List, Union
@@ -25,87 +27,87 @@ def get_calc_buttons() -> OrderedDict:
         order='A1',
     )
     buttons['//'] = dict(
-        value='//',
+        value=' // ',
         symbol='//',
         order='A2',
     )
     buttons['%'] = dict(
-        value='%',
+        value=' % ',
         symbol='mod',
         order='A3',
     )
-    buttons['info'] = dict(
-        value='info',
-        symbol='ℹ️',
+    buttons['_'] = dict(
+        value='_',
+        symbol='_️',
         order='A4',
     )
     buttons[0] = dict(
-        value=0,
+        value='0',
         symbol='0',
         order='E1',
     )
     buttons[1] = dict(
-        value=1,
+        value='1',
         symbol='1',
         order='D1',
     )
     buttons[2] = dict(
-        value=2,
+        value='2',
         symbol='2',
         order='D2',
     )
     buttons[3] = dict(
-        value=3,
+        value='3',
         symbol='3',
         order='D3',
     )
     buttons[4] = dict(
-        value=4,
+        value='4',
         symbol='4',
         order='C1',
     )
     buttons[5] = dict(
-        value=5,
+        value='5',
         symbol='5',
         order='C2',
     )
     buttons[6] = dict(
-        value=6,
+        value='6',
         symbol='6',
         order='C3',
     )
     buttons[7] = dict(
-        value=7,
+        value='7',
         symbol='7',
         order='B1',
     )
     buttons[8] = dict(
-        value=8,
+        value='8',
         symbol='8',
         order='B2',
     )
     buttons[9] = dict(
-        value=9,
+        value='9',
         symbol='9',
         order='B3',
     )
     buttons['+'] = dict(
-        value='+',
+        value=' + ',
         symbol='+',
         order='B4',
     )
     buttons['-'] = dict(
-        value='-',
+        value=' - ',
         symbol='-',
         order='C4',
     )
     buttons['*'] = dict(
-        value='*',
+        value=' * ',
         symbol='*',
         order='D4',
     )
     buttons['/'] = dict(
-        value='/',
+        value=' / ',
         symbol='/',
         order='E4',
     )
@@ -114,12 +116,12 @@ def get_calc_buttons() -> OrderedDict:
         symbol='.',
         order='E2',
     )
-    buttons['*1000'] = dict(
-        value='*1000',
+    buttons['thousands'] = dict(
+        value='000',
         symbol='000',
         order='E3',
     )
-    buttons['\n'] = dict(
+    buttons['end'] = dict(
         value='\n',
         symbol='✅',
         order='F1',
@@ -132,7 +134,32 @@ def get_calc_buttons() -> OrderedDict:
     return buttons
 
 
+def get_operators() -> dict:
+    def multiply(a, b):
+        """Call operator.mul only if a and b are small enough."""
+        if abs(max(a, b)) > 10 ** 21:
+            raise Exception("Numbers were too large!")
+        return operator.mul(a, b)
+
+    def power(a, b):
+        """Call operator.pow only if a and b are small enough."""
+        if abs(a) > 1000 or abs(b) > 100:
+            raise Exception("Numbers were too large!")
+        return operator.pow(a, b)
+
+    return {
+        ast.Add: operator.add,
+        ast.Sub: operator.sub,
+        ast.Mult: multiply,
+        ast.Div: operator.truediv,
+        ast.Pow: power,
+        ast.FloorDiv: operator.floordiv,
+        ast.Mod: operator.mod
+    }
+
+
 calc_buttons = get_calc_buttons()
+operators = get_operators()
 
 
 def get_calculator_keyboard(additional_data: list = None):
@@ -144,9 +171,10 @@ def get_calculator_keyboard(additional_data: list = None):
                 text=button['symbol'],
                 prefix='calc:///',
                 delimiter='|',
-                data=[*additional_data, button['value']]
+                data=[*additional_data, code]
             )
-            for button in sorted(calc_buttons.values(), key=lambda b: b['order'])
+            for code, button in sorted(calc_buttons.items(),
+                                       key=lambda b: b[1]['order'])
         ],
         4
     )
@@ -158,31 +186,30 @@ async def _calculate_button(bot: Bot,
                             language: str,
                             data: List[Union[int, str]]):
     text, reply_markup = '', None
-    if update['from']['id'] not in bot.shared_data['calc']:
-        bot.shared_data['calc'][update['from']['id']] = []
-        if len(data) < 2:
-            record_id = bot.db['calculations'].insert(
-                dict(
-                    user_id=user_record['id'],
-                    created=datetime.datetime.now()
-                )
+    if len(data) < 2:
+        record_id = bot.db['calculations'].insert(
+            dict(
+                user_id=user_record['id'],
+                created=datetime.datetime.now()
             )
-            data = [record_id, *data]
-        else:
-            record_id = data[0]
-        asyncio.ensure_future(
-            calculate_session(bot=bot,
-                              user_telegram_id=update['from']['id'],
-                              language=language,
-                              data=data)
         )
+        data = [record_id, *data]
         text = bot.get_message(
             'useful_tools', 'calculate_command', 'use_buttons',
             language=language
         )
         reply_markup = get_calculator_keyboard(additional_data=[record_id])
+    else:
+        record_id = data[0]
+    if record_id not in bot.shared_data['calc']:
+        bot.shared_data['calc'][record_id] = []
+        asyncio.ensure_future(
+            calculate_session(bot=bot,
+                              record_id=record_id,
+                              language=language)
+        )
     update['data'] = data
-    bot.shared_data['calc'][update['from']['id']].append(update)
+    bot.shared_data['calc'][record_id].append(update)
     # Edit the update with the button if a new text is specified
     if not text:
         return
@@ -195,22 +222,79 @@ async def _calculate_button(bot: Bot,
     )
 
 
+def eval_(node):
+    """Evaluate ast nodes."""
+    if isinstance(node, ast.Num):  # <number>
+        return node.n
+    elif isinstance(node, ast.BinOp):  # <left> <operator> <right>
+        return operators[type(node.op)](eval_(node.left), eval_(node.right))
+    elif isinstance(node, ast.UnaryOp):  # <operator> <operand> e.g., -1
+        # noinspection PyArgumentList
+        return operators[type(node.op)](eval_(node.operand))
+    else:
+        raise Exception("Invalid operator")
+
+
+def evaluate_expression(expr):
+    """Evaluate expressions in a safe way."""
+    return eval_(
+        ast.parse(
+            expr,
+            mode='eval'
+        ).body
+    )
+
+
+def evaluate_expressions(bot: Bot,
+                         expressions: str,
+                         language: str = None) -> str:
+    """Evaluate a string containing lines of expressions.
+
+    `expressions` must be a string containing one expression per line.
+    """
+    line_result, result = 0, []
+    for line in expressions.split('\n'):
+        if not line:
+            continue
+        try:
+            line_result = evaluate_expression(
+                line.replace('_', str(line_result))
+            )
+        except Exception as e:
+            line_result = bot.get_message(
+                'useful_tools', 'calculate_command', 'invalid_expression',
+                language=language,
+                error=e
+            )
+        result.append(
+            f"<code>{line}</code>\n<b>= {line_result}</b>"
+        )
+    return '\n\n'.join(result)
+
+
 async def calculate_session(bot: Bot,
-                            user_telegram_id: int,
+                            record_id: int,
                             language: str,
-                            data: List[Union[int, str]]):
-    queue = bot.shared_data['calc'][user_telegram_id]
+                            buffer_seconds: Union[int, float] = 1):
+    # Wait until input ends
+    queue = bot.shared_data['calc'][record_id]
     queue_len = None
     while queue_len != len(queue):
         queue_len = len(queue)
-        await asyncio.sleep(2)
+        await asyncio.sleep(buffer_seconds)
     last_entry = max(queue, key=lambda u: u['id'])
-    # Remove user queue
+    # Delete record-associated queue
     queue = queue.copy()
-    del bot.shared_data['calc'][user_telegram_id]
+    del bot.shared_data['calc'][record_id]
 
-    record = None
-    text, reply_markup = '', None
+    record = bot.db['calculations'].find_one(
+        id=record_id
+    )
+    if record is None:
+        logging.error("Invalid record identifier!")
+        return
+    expression = record['expression'] or ''
+    reply_markup = get_calculator_keyboard(additional_data=[record['id']])
 
     # It would be nice to do:
     # for update in sorted(queue, key=lambda u: u['id'])
@@ -222,30 +306,35 @@ async def calculate_session(bot: Bot,
         if len(data) != 2:
             logging.error(f"Something went wrong: invalid data received.\n{data}")
             return
-        if not text:
-            record = bot.db['calculations'].find_one(
-                id=data[0]
-            )
-            text = record['text'] or ''
-            reply_markup = get_calculator_keyboard(additional_data=[record['id']])
         input_value = data[1]
         if input_value == 'del':
-            pass
-        elif input_value == 'info':
-            pass
+            expression = expression[:-1]
         elif input_value in calc_buttons:
-            text = f"{text} {calc_buttons[input_value]['value']}"
+            expression += calc_buttons[input_value]['value']
         else:
-            pass  # Error!
+            logging.error(f"Invalid input from calculator button: {input_value}")
     if record:
         bot.db['calculations'].update(
             dict(
                 id=record['id'],
                 modified=datetime.datetime.now(),
-                text=text
+                expression=expression
             ),
             ['id']
         )
+    if expression:
+        text = bot.get_message(
+            'useful_tools', 'calculate_command', 'result',
+            language=language,
+            expressions=evaluate_expressions(bot=bot,
+                                             expressions=expression,
+                                             language=language)
+        )
+    else:
+        text = bot.get_message(
+            'useful_tools', 'calculate_command', 'instructions',
+            language=language
+        )
     await bot.edit_message_text(
         text=text,
         update=last_entry,
@@ -275,7 +364,14 @@ async def _calculate_command(bot: Bot,
         )
         reply_markup = get_calculator_keyboard()
     else:
-        text = 'pass'
+        # TODO: make a new calc record
+        text = bot.get_message(
+            'useful_tools', 'calculate_command', 'result',
+            language=language,
+            expressions=evaluate_expressions(bot=bot,
+                                             expressions=text,
+                                             language=language)
+        )
     await bot.send_message(text=text,
                            update=update,
                            reply_markup=reply_markup)
@@ -438,7 +534,7 @@ def init(telegram_bot: Bot, useful_tools_messages=None):
             types.datetime
         )
         table.create_column(
-            'text',
+            'expression',
             types.string
         )